Aluminum "flashing" should work, find it at Lowe's or Home Depot. About .005 thick as I recall. It comes in a roll-various widths. Look for it in the roofing dept.
That,s what I use, except that the stuff I bought- from a roofing supply place- is about .012-.015" thick and pre[painted white on one side and blue on the other. The blue isn't close enough to sky blue to suit me, but a coat of flat interior latex paint fixed that. 24" seems to be the maximum width X 50' roll.
____________________ Fix it again, Mr Gates--it still works!"