No, they are vibration dampening material (rubber / silicone) and only 2mm thick. They are the right shape but not rigid or thick enough to be used as mounting adapters. Something like this:120mm to 140mm mounting adapter for 140mm fans with 120mm mounts
I've made these before and they work great. Only need a drill press and table saw .. and a 135mm hole saw.
If person was doing 50+ it would not take long per adapter, but it's very time consuming when only making a couple.Takes more time setting up for each step when only doing a few than actual cutting / drilling time. First setup to cut the 135mm holes, then cut squares w/ 135mm hole, then drill 104.5mm spacing holes & countersink, then drill 124.5mm spacing holes.
I have not seen anything similar being marketed. Wish someone would. Nothing trick or expensive.