As far as I can acertain from the design you have, you Can comfortably fit Drums Bass Gtr and a Keyboard Player in there. Not sure what your band consists of.
That Fact that your space is 24 feet long is a very good thing. Average Small Performing Stage is 24x20 and that fits a five picce with comfortable room for the Lead singer to run and clown.
With The Treatments in there. You can do a variation of that drawing...By Losing the angles on the right live room wall and angling the top wall in like fashion.
Here is the Plan John did for me.
Bryan Giles