I started by cutting a 20 cm circle out of the piece of MDF with a router and circle jig. Sanded down the rough edges and spread woodglue all over the disc. Then I placed the disc on the back of a new sheet of sandpaper, put another piece of wood on the other side and clamped the whole thing very tight. After 4 hours, I removed the clamps and cut the sandpaper down to size with a Stanley knife. The resulting sanding surface looks great, no bubbles or ridges in sight.
Some months ago I picked up a discarded weed stripper. The 230V motor still ran but the stripping wheel thingy was all worn down. So i took everything apart and kept the things that might come in handy one day! The motor includes a reduction gear box that also turns the angle of rotation by 90 degrees and makes it easier to mount.
I took the worn down stripping wheel, drilled 4 holes spread along the axis and screwed it to the back of the sanding disc. Now the disc can easily and securely be fastened to the motor using the original parts as intended.
A sheet of laminated particleboard will be the base for the sander station. I have been working on a mounting block to hold the motor and gearbox firmly in place but have run into some problems. I used a couple of metal tie wraps but they fail to reliably secure things to the wooden base. Another strategy is needed...