I'm now thinking along @RickyX lines that a piece of rubber or foam block, cut to size/shape with velcro top and bottom could work nicely.
what about correct size P clips mounted on the round bar of the subframe either side with a flat bit of carbon or aluminium bolted between them with the correct size hole and grommet in it!
Still a bodge. Why not do the job properly and either modify the subframe correctly or get the right subframe. You can buy a bip subframe for £100 and an aluminium race one is under £200. They're a nice bike, worth more than a lash-up surely?
“When you’re a carpenter making a beautiful chest of drawers, you’re not going to use a piece of plywood on the back, even though it faces the wall and nobody will ever see it. You’ll know it’s there, so you’re going to use a beautiful piece of wood on the back. For you to sleep well at night, the aesthetic, the quality, has to be carried all the way through.” ― Steve Jobs
Ah, Mouse Man - his work shop is near us, it will cost more than a bespoke carbon Fiber bracket though
CR, this is what was used on mine with the altered bip subframe (you have seen before). I cant claim credit for it but I was told many years ago as an engineering apprentice that a good engineer can do what anyone else can do but for less than a tenth of the price. This works and can be done in a matter of minutes. If you want to restore it fully as a mono (assuming that is what the bike originally was) then obviously the subframe needs replacing but in the meantime.....
