You got really nice pictures of the result. Much better representation than any attempt I’ve made so far.
I use DC-Fix on several lights, but I don’t really understand using it on a thrower when you can get the same beam out of something much smaller. Instead of a Convoy L4, you could do this with a Convoy S2. I think the items I’m currently using DC-Fix on are: ZL SC52, CNQG brass beauty, Convoy S3-XP-G2, Convoy S7-219B, and BST-wide. And the ZL H52Fw looks similar due to its frosted lens. The BST is sort of an odd choice for diffuser film since it has quite a bit of throwing potential, but I went for wide-spectrum with lots of floody lumens instead.
When I put diffuser film on a removable lens, I find it easiest to put it directly onto the sticky side of the film, cut around it with a thin, sharp knife, and then just leave it on. Maybe push out some bubbles first, but mostly just pop the lens right back in. No need to remove it and trim the edges further. If I can’t remove the lens though, things get a little more tricky. Then I need to find a stencil of just the right size to cut around, like a coin with the same diameter as the exposed portion of the lens.