Are you talking about tig welding a small piece to cover the slot on the top or do you also cover it on the bottom where the rotors are so you don't create two small pockets for air flow to hang up?
I was thinking of epoxy and then you can fill the slots, but I wonder why JB Weld or Liquid aluminum wouldn't hold up if you prep it right? I was thinking about making something to put on the inside to hold the epoxy in there till it hardened but then be able to remove it without having anything in the rotors path. I don't know if paper would do it, but you aren't gonna damage anything by having a small tiny piece of paper get shredded and burnt up and blown out the exhaust.