Both of those gaskets can be made to "grow" with just a small amount of tugging on them.
Even though you're tempted, DON'T TUG ON THEM!!
Just push them on so they are in a neutral state as applied, no stretching, no shrinking.
Now, on the meeting of the gaskets at the corner posts, run the bottom gasket out to the very end to but up against the corner post, then cut off the top frame gasket so that it has about 1/8" of excess length, such that it presses down onto the bottom gasket when installed and the top frame screws are installed (the corner posts should already be properly tight).
If you wish, you can use a bit of black silicon RTV caulk to seal them both together (I usually do). That caulk can be found at Home Depot/Lowes. I would avoid the NAPA stuff - I've never had much luck with NAPA silicon and it's hard to find black Permatex at Napa.