The Beetle B16 gearmotors are nice, but frustratingly hard to build motor mounts for. They have a "double-D" face profile that requires drilling a large hole, but the screw holes that hold it in are so close to the big hole that it's hard to get right. The easiest way is to buy pre-made mounting plates from Kitbots.com. I'm going cheap on this and, frankly, I'd have to re-design a lot to fit those plates. So after some trial and error today, I found something that works reasonably well.
I have a dead B62 gearbox from a previous competition -- and it has the same hole pattern as the B16. So I took the motor's faceplate off and popped the shaft out of it. Then I tapped the shaft hole with an 8-32 threading tap (it's a little loose but not too much) and stuck a short 8-32 bolt in it.
I'm mounting these in 1/4" thick Aluminum bar. When I go to drill the mounting holes for the motor, I start by drilling and tapping an 8-32 hole centered with the motor (i.e., where the shaft would protrude) and then screw my salvaged-faceplate-plus-bolt into that hole:
Then I can use the existing holes from the faceplate as guides for drilling. Once those are done, I take off the faceplate and bolt and switch to a 9/16" drill bit, which I sink just a little into Aluminum bar -- just enough to accommodate the "double-D":
Finally, I make the center hole a little bigger so the motor shaft doesn't touch the sides and voila! Integrated motor mount:
Update, for those frustrated souls scouring the Interwebs looking for enlightenment on mounting these motors: I highly recommend using counter-sunk M2 machine screws to hold the motors in. It doesn't show on this page, but I switched to counter-sunk screws later. Well worth the trouble!