Use 3/16 thick plastic and router or cut the ends at a 45 degree angle. And on one peice I
weld a clear plastic strip 2" wide so it overlaps on the back.
OR get the right size plastic... Lexan comes as long as you need.
Or go to a flex face, that is what I'd do.
We've used changeable tracking used for changeable letter signs. We have used the bottom track and cut 2 pieces and used IPS weld on. We welded one piece of tracking on the edge of piece one and welded another piece of tracking (flipped opposite of how the first piece was welded) on piece two. Push the pieces together in the box and the 2 pieces of tracking interlock creating a nice tight seal along the joint of the 2 panels.
use roll stock - way easier - you'll never be happy with a join.