[MrForbes]

If you play with the plywood parts some you might change you're mind, they seem pretty strong.

But setting up the saw and jigs to make all the parts the same size seems to be a chore. I'll look into what it would take to make them out of welded steel, too.

I like the idea of redesigning it to use the rubber stoppers that already have a hole in them.
The wheels might have to be a bit smaller than this design's 4" diamter to work well though, as most stoppers seem to be 1" tall, and the current design requires the rollers to be about 1.25" tall