I'm going for a 4wd wedge pusher, and part of the plan is that it should work upside down just as well as it does rightway up, with a switch on the controller to invert the drive direction.
I spent some time in FreeCAD last night and came up with this:
The chassis is made of 2 identical halves that bolt together, and the wedge at the front is free to pivot so that it always touches the ground. The motors are just held in place by the chassis being bolted together, there's no actual "motor mount" as such.
I also plan to add some horns that stick up on "top" (whichever side that is) to stop other robots from just driving over the top. Basically a double-ended horn that rides in a slot inside, so that the weight of the robot pushes the top horn up, and the bottom horn rides against the ground.
So far I'm estimating 160g without any horns, and that's at some pretty dubious 3d print settings (5% infill, only 1 perimeter) so I probably have to design the chassis a bit more sparsely.