How much room is there between your topper/canopy and the back truck window?
Expanded metal comes in many sizes, but it will all need to be framed to be effective. The thickness and design of the frame and how its mounted to what ever its mounted to, will be the issue. You can just cut the expanded metal over sized and bolt it to the canopy/topper with bolts and self locking nuts, but you will have to remove the canopy to do it. Expanded metal is generally not cheap, and comes in 3' x 8' or 4' x 8' sheets. Some places don't like to sell just a part of a sheet. You will want to see what is available locally to you and determine how large of a piece you will need. Gene