I decided to make these properly rather than bodge something. I bought 10cm square stainless steel 0.3mm and 0.1mm thick. I avoided aluminium as it's compressible when you tighten everything up.
The step metal drill bit was perfect for the job. I held it by hand to do the small holed then screwed it to the wood for the big hole.
If I got it off center I used a diamond bur to enlarge the hole on the correct side.
I used the dolly to make sure it was completely flat around the holes.
FYI I have a new thermostat that extends a lot so have only the thinnest round shim installed. I expect as it ages I'll remove an oval shim and put the same size back as a round shim.
