Here's the factory Valmet buffer.
Valmet Ejection Port Buffer
They are even more expensive on Gunbroker.
Note that the buffer is THICK. It attaches to the back of the ejection port on the top cover. I think it actually has a metal clip that snaps it on and keeps it there.
I have considered talking to BlackjackBuffers about making some of these, and he would need a sample of one to get started. I worked with him to create the Uzi and FNC buffers, and at taht time all he needed was a shape and thicknes. Perhaps we could just design one from scratch. A metal clip would be easy to find I would think from some place like MSC or McMaster. The thickness makes it "semi permanent" if you get my meaning.
For your application, you may want to see how it dents before investing in one. My dents are honestly a small crease about as wide as my fingernail, and they have reloaded and fired no problem. The 2nd firing tends to act as a fire form, and other than cosmetics it doesn't seem to be a big deal. Now if you don't want to tweak the ejector on your build (aka grind and reweld it) then a buffer makes sense.