In the Supernatural series, the show takes certain liberties with arcane lore, sometimes to make it more interesting or socially relevant, other times because it is visually more appealing or both. In the case of Devil's Traps or Keys of Solomon, a circular symbol used to imprison a demon within its confines, their tendency is to place it on the floor, usually hidden in some fashion. They have occasionally been placed on the ceiling and are no less effective for being drawn there.
##Supernatural (TV Series) Lore
The trap can be painted, drawn in almost any substance from paint to blood, though more durable media make it harder to erase. Hiding the trap does not make it less effective and they can be created in media that can't be easily seen such as luminescent paint visible only under black light. The demon must be standing within the confines of the trapped region.
If the ceiling or floor is slanted is less relevant than the fact they are within the confines of the trap. Painting one on a wall, however, should not work. There are examples of traps used in exotic ways (such as a bag covering the head of a possessed human) whose efficacy is questionable but perhaps due to effects such as possession, it may make sense since the demon resides in the mind of the possessed, thus binding them in place and preventing escape.
The reason I suggest walls don't work have nothing to do with occultism and everything to do with practicality. If they did, there would be no reason to ever not paint one's home to include every surface with traps of all kinds. Bobby Singer had several through out his home, but did not cover every wall with them. There must have been some limiting factor even if we don't know what it was.
##Classic Occult Lore
In keeping with rituals and lore, the floor or ceiling seem to be the only choices because they are used to bind the demon to the location and can be linked to the primal sources of Heaven or Hell. The lore surrounding such circles has almost always had them drawn on floors to keep demons at bay while negotiations took place for the demon to perform a service.
Once the demon or djinn had given its word, they could be safely released until they completed their task, then they would be allowed to return to Hell or their dimension of origin. Unlike the Supernatural Devil's Traps, in ancient demonic lore, a circle could be created for the summoner as well which would keep the demon at bay in case the original binding circle in which the demon was summoned failed. The hope was a demon would leave before the summoner starved to death.