That's actually some very nice work! I think with the body not getting any smaller I'd stick with the 4*18650 format with the SRK since the overall capacity and runtime would be far greater. Even just 2*18650 would give more capacity than a single 26650. It's an awesome idea though and you executed it very well!
I think your positive contact mod would be great on all SRK lights as the etched on contact trace doesn't look too durable. Your solution would hold up fantastically and last nearly forever.
Nice work. Looks like you have enough room to add 4 springs so you use 18650’s without changing the plate. They may need to be a little off center to not hit the 26650.
I was thinking of doing something similar to the Rook. The center section of the Rook can be used to power the light. I wanted to have a hole in the center of the bottom plate to accommodate a longer cell, but still be able to use AA’s. I couldn’t find enough room. So now I’m just going to throw a free floating plate in the bottom after boring out the center divider section for a fatter cell. Wanted to keep the battery versatility. I think I like your approach better for higher current flow.