Trying to remember the name of a story from the 50s or 60s. A solitary man's slovenly habits (and the crusty rag under his bed) give rise to a sort of immaterial malevolent creature, which creates the form of a woman from random odds and ends to lure him to his death.
You might be conflating two stories - I also thought of "The Rag Thing" as mentioned in user14111's answer. But, the "immaterial malevolent creature" "creating the form of a woman from random odds and ends" is very reminiscent of the ending of the 1952 story "An Egg a Month From All Over" by "Idris Seabright" (Margaret St. Clair).
The creature in this story does not start out as a rag under the bed (it hatches from an egg), but it does start out as immaterial, and it does create the form of a woman to lure a man to his death.
The faint pecking from within the egg grew louder. The dark fissure on the pale blue-green background widened and spread. The halves of the shell fell back suddenly, like the halves of a door. The egg was open. There was nothing inside...
...The mnxx bird imago, left alone within the cabin, flitted about busily.
creates the form of a woman from random odds and ends to lure him to his death
In its flittings in the cabin during his absence, it had managed to assemble for itself a passable body. It had used newspapers, grapes, and black wool from the afghan as materials. What it had made was short and squat and excessively female, not at all alluring, but it thought George would like it. It held the nail file from the drawer in its one completed hand...
...Her face was only a blur; there the mnxx bird had not felt it necessary to be specific. But she moved towards George with a heavy sensual swaying; she was what George had always wanted and been ashamed of wanting. She was here. He had no questions. She was his. Desire was making him drunk....
...When it was finished, George lay on the sodden carpet flaccidly. His eyes were gone, and a lot of his vital organs....
...The mnxx bird, on the fine strong wings it had plaited for itself out of George's head hair, floated out into the night.
Both of these stories are included in the same anthology, Fifty Short Science Fiction Tales, so you could certainly have read them at the same time.
This is a long shot.
Parts of your description are slightly reminiscent of Donald A. Wollheim's 1951 short story "The Rag Thing", first published (as by David Grinnell) in The Magazine of Fantasy and Science Fiction, October 1951, available at the Internet Archive. However, the rag-creature is not immaterial, and it does not lure its victims to their death by creating the appearance of a woman. Moreover, it's not the victims' but the landlady's slovenly habits that give rise to the creature.
The landlady and the rag:
In a way, though, it is quite possible to hold Mrs. Larch to blame for everything that happened. Not that she had what people would call malicious intentions, but just that she was two things practically every boarding-house landlady is—thrifty and not too clean.
She shouldn't have been in such a hurry to turn the heat off so early in March. March is a tricky month and she should have known that the first warm day is usually an isolated phenomenon. But then you could always claim that she shouldn't have been so sloppy in her cleaning last November. She shouldn't have dropped that rag behind the radiator in the third floor front room.
As a matter of fact, one could well wonder what she was doing using such a rag anyway. Polishing furniture doesn't require a clean rag to start with, certainly not the rag you stick into the furniture polish, that's going to be greasy anyway—but she didn't have to use that particular rag. The one that had so much dried blood on it from the meat that had been lying on it in the kitchen.
On top of that, it is probable that she had spit into the filthy thing, too. Mrs. Larch was no prize package. Gross, dull, unkempt, widowed and careless, she fitted into the house—one of innumerable other brownstone fronts in the lower sixties of New York. Houses that in former days, fifty or sixty years ago, were considered the height of fashion and the residences of the well-to-do, now reduced to dingy rooming places for all manner of itinerants, lonely people with no hope in life other than dreary jobs, or an occasional young and confused person from the hinterland seeking fame and fortune in a city which rarely grants it.
So it was not particularly odd that when she accidentally dropped the filthy old rag behind the radiator in the room on the third floor front late in November, she had simply left it there and forgotten to pick it up.
Chemical reactions and the birth of life:
It is hard to say what is the cause of chemical reactions. Some hold that all things are mechanical in nature, others that life has a psychic side which cannot be duplicated in laboratories. [. . .] Heat and moisture and greasy chemical compounds were the sole ingredients of the birth of life in some ancient unremembered swamp.
The rag thing claims its first victim:
Anyway Skelty was found dead in bed the next morning. Mrs. Larch knocked on his door when he failed to come down to breakfast and when he hadn't answered, she turned the knob and went in. He was lying in bed, blue and cold, and he had been smothered in his sleep.
There was quite a to-do about the whole business, but nothing came of it. [. . .] Of course the body was unusually cold when Mrs. Larch found it, as if the heat had been sucked out of him, but who notices a thing like that? They also discounted the grease smudge on the top sheet, the grease stains on the floor, and the slime on his face.