I have an older style 125G that I am setting up on a DIY stand made of 2X4's. It is a glass aquarium and has raised rim base. I am guessing that by "raised rim" you are talking about the base's frame supporting the glass bottom that elevates the glass bottom about a 1/2". Anyway, I had it filled for a week to check for leaks, no gravel or decor just water. The stand is just a 2X4 frame with no "top" on it yet. I did check to see how the glass looked and it appeared not to bow. Since it is raised I would care to guess leaving the top of the stand open would be OK. I however am going to use 3/4" plywood for a top for the stand. I plan on using 1/2" foamboard insulation to fill the gap and cushion any possible bow because it is going to have a deep sand bed and alot of rockwork for a rift lake setup. If there wasnt going to be alot of rocks I might not worry about any bowing of the base and go with an open type.

Most all stands are made like you are describing. The main area of support is around the edges of the tank. As long as the stand sits level I wouldn't worry about it.

Lisa