Imagine a square sleeve. Split it diagonally. Incorporate 608 bearing mount points so you can have bearings riding on all four sides. I would do a 2-1-2-1 pattern for the bearings. You can zip tie the halves onto the square tube or spring load them or screw them together.
This is assuming that you have a 3D printer already or have access to one.
This is assuming that you have a 3D printer already or have access to one.