I've got a rock and roll bed with no isofix. At first I was using isofix adaptors
Amazon.co.uk plus the seat belt. They worked but it was a bit meh. I did feel the kids were safe though. Almost triple belted.
I've now taken the the front seat cushion off the R&R and glued and bloted the isofix bracket to the bed (drilling through the bed frame and using M8 bolts). I know people will say not crash tested blar blar but I can safely say the level of impact we would need to move those brackets we would long we dead to worry. I also still use the standard seat belts on their seats.
I'm driving my kids so a bit easier than grandkids but it's worked well for me if you don't mind a bit of drilling though your bed frame.
If not, find a good fab shop.