[Bushkill]

These screens work best for tanks with internal overflows like some of the SW tanks that are drilled. Is this canister filter intakes and returns you're working with as opposed to an HOB?

 

[Bushkill]

I made one for a SW tank years ago and cut a "U" into the back edge of one. The real issue was how to support it. If the screen is of any size, it becomes a weak point and sags. An option may be to cut it short as you're thinking and possibly crazy gluing strips of acrylic to the back edge that will take it to the tank rim, leaving openings where you need them?