We know from the film's narration that the Jaegar project was the most massive engineering project in the history of mankind. We also know that the Jaegers initially proved very successful against the single incursions, with only one lone scientist predicting that there would eventually be double incursions.
In addition, we know that The Wall was being constructed as a long-term solution, and was also a massive engineering project that spanned the globe. Presumably, the resources and effort needed to build & maintain a defensive "fortress" around the Rift were calculated to be higher than the existing Jaeger or Wall defense. Had additional resources been available, it's more likely that they would have been used to construct additional Jaegers to bolster the existing defense.
Besides the massive logistics involved in building an underwater facility of that size, you would also have to account for the defense of the project while under construction. Say you get the fortress 2/3 done, then a Kaiju pops out and gets one good swipe across it despite the efforts of any on-site Jaegers. Now all that effort - as well as billions of dollars & any invested resources - are gone and you're back to square one. Speaking from a loss/gain perspective, it's not a very efficient solution compared to others.
The Wall, on the other hand, was believed to be a much better solution. The world's leaders & experts had no way of knowing it would be that ineffective against the Kaiju, and everyone seemed to believe that it was a fail-safe answer to the problem.