Are 26mm rings readily available? can you cheat and use 1"?
Talley makes 26mm rings in their vertically split 'Signature' line. You might have to call to get them.
I have a small supply of NOS vintage 26mm rings from the last time I went through this with a vintage Swarovski.
Some 1' rings can be reamed from 1' (25.4mm) to 26mm with an adjustable reamer.