Thanks for the info and thoughts about the safety, but I’ve used these levers with these bars for well over a year with those rubber shims without any issues. Then I switched to different bars, but I’ve switched back to this riser bar setup but the rubber shims I had before I’ve used elsewhere.
The bars are Nitto For Shred, and the levers are Origin8 cross levers. I don’t remember what size exactly the version of the levers are because I bought them in a shop and they weren’t labeled, but the clamp area is a bit bigger than the grip area diameter where I mount them.
I don’t have any spare flat tubes at the moment but I may go that route.