Steel tape, 0.4mm and 0.2mm far too thick and stiff. In the end I lined the mount cups with 0.05mm CS4 Steel Shim, £1.95 off eBay for an 6 x 3 inch piece. Cut with scissors and glued it in with epoxy, filed and sanded it smooth. In the Scope I Drilled 24 holes, 3mm wide and glued in 2mm x 1mm magnets. Works quite well. The tell will be when its fully assembled.