Jörg how do you make your bearing swivels? I have designed and commissioned some blocks made of a stack of 3 ball race bearings packed within a bored-out case and retained between internal-style circlips, with the shaft also secured with external circlips. It should handle the strong side load and rotate freely. Yours looks more straightforward from your videos, but I can't tell how it's done exactly.
Then I cut the steel rod to about 12 cm and weld it to the frame.
Then I cut a 12 cm length from the aluminum tube.
I press in the ball bearings in the vise. Be careful not to cant the bearings. No need to heat the tube, but oil it lightly so the bearing slides in OK. The bearing will sit very tightly in the tube, not glue is necessary.
I press one of the bearings a cm in, so it does not cause a problem with the welding seam. The other one can be flush.
Next, I slip the tube with the bearings over the steel rod. This may require a few taps with the wood hammer. I usually smear a little epoxy on the rod beforehand so the bearings won't slip later.