Yep, these things get brittle over the years too, and become even easier to crack.
The factory usually sealed the four screws with roof sealant, so you may want to consider doing that too. It's a pain to dig it out when you need to change the cover, but it keeps the screws from rusting away, since those holes will fill up with water.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR