Geoff, Try finding a local roofing contractors. He may have a scrap of Grace Ice and Water Shield he will sell,or give you. Aluminum piece will work, however be very careful,of screw length, as you mentioned and don't use any power tools or grinders that may cause sparks on the hole.I also never planned to cut to access the pump. However, when my pump died yesterday, I decided it just wasn't worth dealing with tank removal. That is not fun. The fuel lines can also be very delicate and easy to damage. Plenty of problems could arise from removing the tank, so I'll be cutting.
Having spent a fair bit of time working under vehicles, I feel that it's not worth it unless absolutely unavoidable. Why risk personal injury if you don't have to.
And the ice & water barrier sounds like an excellent sealing solution, but it could be hard to acquire a small piece, as it's sold in large, expensive rolls. I'm pondering a cover fashioned from a piece of aluminum sheet. Then secure it with short self drilling screws (with the tips then filed down). Waterproofed with a bead of silicone.