Ok, almost everything's in place now. I'm aiming to get a final CAD diagram down by sometime this weekend. I do have a conundrum however: how do I mount the wheels? So I'm using 18v drill motors (should be simple enough). They have a 12mm outer (right handed) thread, which appears to be M9, and an inner left-handed M5 thread, and the screw protrudes by about 9mm when fully inserted. The screw head is 9mm in diameter. So basically my motor shaft has a 21mm (25mm at a push) length.

My wheels, however, which I suppose I could've been a little more selective with, are around 34mm width including the hub, with a 13mm diameter hole in the middle. The full wheel diameter is 100mm.

I thought about putting a coupling nut in the middle (I've seen a video where they heat them up and drive them into the centre of the wheel, melting the plastic around) but I can't for the life of me find an M9 coupler. I still have the chucks from all the drills, but they don't seem to look hackable enough after taking one apart today. I also thought about getting a new, longer screw and just clamping the wheel on with some washers, but M5 reverse-threaded bar or screws seem very rare, and even more so for locking nuts if I'm using bar. Is it worth getting new, thinner wheels (it would be a bit of a waste of these)?

TL;DR: My wheel holes are too wide and my motor shaft won't reach through, what do?