I recently got some cheap wooden dollies with caster wheels for moving plant pots, furniture etc.

The boards were supposedly rated for 100 kg, but when I did put something fairly heavy on them, they just collapsed silently because the cheap-ass soft plastic mounts for the casters gave out. The casters themselves are rather sturdy (Wagner RO0726, with an 8 mm steel pin), so I designed new mounts and epoxied them into the original holes, and hey presto, new workable dolly boards.

If you want to mount the casters into something else, you need to drill a 12 mm hole, 16 mm deep.