what do you mean with rounding off?

basically, the wheels are made like this: drill a 5mm hole into a 10mm thick piece of hdpe.
attach jigsaw to the table, screw the sheet into the table through that hole.
turn the sheet around the hole, and against the jigsaw to saw everything off that's further away from the hole than the jigsaw.

screw on bike tire and widen the hole to fit mount in.

so it's all the same thickness.
luckily i have found someone now who's gonna making me some hubs, basically just nuts with plates on, but made from one piece and not welded together. seems to be not that hard to do if you have a lathe...