RE: Sukhoi Su-26
That looks like it was just light ply or balsa stager glued then cut round and glued to the cowl. In my case I used one of the plastic fake engines and braced it with light ply and made a ring mount at the top and glued it in. If you want you can do something like in the photo and cut out 1/8 slots in the over lap. In his case he just has the engine section opened, that's all that is required for cooling. I used gorilla glue to install mine so it would expand aND FILL IN ANY blems in the glass cowl.