The problem you're likely to have is that the topper had a curved, all glass lift gate. You may have to get creative with weather stripping to get a flat piece of material to seal.

This is the reason why when I buy a truck topper I always get the kind with the flat rear glass surrounded by an aluminum frame. Much more difficult to break and if it does break, any glass shop can repair it cheaply.

Those all-glass curved lift gates look great but they are hella pricey to replace. It's not like an SUV where you can source a replacement from a junkyard.