Sounds like an excellent plan! I've really struggled trying to find the perfect lid. On one, I use 1/3 of a drum, cut right at one of the ribs, which overlaps the top bead of the drum. It works well, and has a lot of room inside, but it is very heavy. In order to use a Weber lid, most drums are just a hair too big, and re-shaping the Weber lid results in the loss of the baked-on enamel- not good. I've also tried several large SS bowls, but haven't found one the right size.
I've thought about making one the way you are, and it should work very well- if you can get a good seal. Air leaks at the lid make it very hard to get that low-n-slow burn the UDS is famous for. Pictures- PLEASE!
I'm very intelligent, but I mask it by doing stupid stuff.