If you're insistent on one belt run per side, I think the simplest belt path is
this. You can see the real thing
here. I personally prefer
this system, where there is no need for idlers.
Some tips and tricks for this system:
1.Make sure your pulleys are large enough. We had 15t pulleys and 9mm belt in 2011 and the belts ratcheted pretty badly. Since then we have used 24t pulleys and 15mm belt with no problems.
2. I assume those windows are for internal belts. I'd recommend covering them with 1/16 lexan to prevent debris from getting inside. You can see how we did that if you look closely in the pictures above.
3. Think long and hard about how you are going to weld this. We have moved away from welding in recent years because it is faster for us to gusset. If you are going to weld this, I'd suggest using the belly pan and some gussets as a jig for the welder to keep everything square during welding and to make sure everything is assembled properly. The belly pan can be welded directly to the frame and the gussets can be removed at the end.
4. Also think long and hard about how you are going to do your bumper rail. You can weld them, and if you do, I again suggest you mount the pieces in place first using some 8-32s. Alternatively, you could use rivet nuts to mount all the pieces together like we did
here.
5. You need a way to retain your axles. We put a snap ring on the inside and we tap the ends of our axles and retain the wheels with a bolt and washer, like we did
here.