Nut-Bolt from hardware store, ask for SS or chrome plated. Just 5mm more length than transformer height is okay.
Rather than round plate buy round flange of lesser diameter from machine shop, drill central hole or 3mm thick 2" wide iron strip with hole in center from welding shop. Length of strip lesser than diameter of transformer. Get its rough edges smooth and burr removed on grinder at same shop.
For rubber washer, piece from car tube works, get cut round from cobbler. It works and beats temperature of hot running transformer.
Good luck, take care keep all metal surfaces electrically isolated from surrounding.