My first problem was that the distance between the aluminum extrusions was too wide. Turns out you must take into every little dimension when CADding parts. Since I normally work with things that have infinite tolerance (code/math)...go figure -_- The solution wasn't too tricky; I made two plates that went on top of the extrusions that connected two more extrusions on the respective insides.
This solution introduces more torsional flex, but to what extent is unknown so far. It seems to pass a rudimentary twist and sit test, though.
The second issue dealt with how my rear wheel was installed. Since I had the wheel essentially slide into the space between the aluminum extrusions, I couldn't tighten the bolts if I first installed the extrusions onto the chassis.
Here's to hoping I tightened the bolts enough such that the wheel won't fall off during operation!
I also had a slew of smaller issues, like incorrectly measuring dimensions and not accounting for dimensions and the like.
Because the axle diameter isn't 5/8"; it's really 10mm!
Because T-nuts have finite size
Because axles have finite length and need to be secured...somehow.
That's all for now! Hopefully I can have a rolling frame (with motors!!) by the end of MLK.