A few things you might try...
1) a second layer of rip stop on the sides to diffuse the light better. rip stop is really thin.
2) Go get some large diameter shop lights so you have less of a point source light.
3) get a piece of white foam core and attach one edge to the back corner of the tent and angle it out from the tent at 45-degrees. Now aim your lights into the foam core rather than into the tent. This will give you a light source the same size as your foam core.
Let us know how it goes!