[Aren Siekmeier]

An easy way to get a lot of wrap is to add a chain run. Keep the one between the front two wheels, have one just between the back two, and then only drive one wheel (usually the center makes the most sense) with a chain run off the gearbox. You can also do as Eric described. Then all sprockets have ~180 degrees of wrap (plenty if you don't have too much slack) and tensioning gets easier.

As Chris said, the diagonal cuts in the corners probably won't hold the frame square as easily. They will also be a bit harder to cut exactly how you want, so I would suggest making the interface square, butting them together something like this:

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Also, be careful with how low the chains between the wheels hangs. It looks pretty close to ground, and this leads to more friction, more wear, and a stray bolt on the ground could cause the chain to jump. Either bigger wheels or smaller sprockets on the wheels would help.

You could maybe replace the 2x1 tube with 1x1 to make it lighter since you don't need the space for bearing blocks, but 2x1 will certainly be rigid enough. I assume it's 1/8" wall.

And ditto on the gearbox support.

Looks good overall!