I have not replaced it with a flat sheet but I did replace the bubble after hail shattered mine. I found the exact replacement online. I ordered the one with the predrilled holes in the flange, sorry I can't find the link but it wasn't hard to find. Just measure the overall length and width.
I removed the old bubble and used a putty knife to get under the old dicor and butyl tape and pull it off. Then I used a bit of mineral spirits to clean the area real good. Laid down a double wide strip of butyl tape and screw the new bubble down, being careful to not overtighten and break the plastic. Then I used a tube of self leveling dicor to cover the seam edge and all the screws. I went ahead and used the entire tube to cover it all again.
Easy job, took a couple hours maybe, start to finish.
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