The gutter guard is essentially just expanded metal mesh. Hardware cloth works even better, but you need to buy more of it typically so the gutter guard is cheaper.

Cut the gutter guard into two pieces that span a little less than the width of your tank. This will be come the actual wall between the two sides. Stack one on top of the other to give the mesh the proper height. The pine 1x1's sandwich the mesh on both sides and hold the top over the bottom. Bolt the sandwich together on each side. So now you have two pieces of metal mesh (on over the other) that are held in place on each side with two 1x1's. It is important to make the total width just slightly wider than the tank width, so that when installed the mesh kinks a little. The kink acts as a spring, pushing against the side of the tank. Since the mesh comes in two pieces, and the gerbils tend to climb this thing, they can get their feet stuck if the two halves of the mesh are not wired together. I just use a few twist ties to hold them together.

When installing into the tank, care should be taken for the gaps on the top and bottom. Make sure the gaps are filled somehow. At the top we typically use duct tape, and at the bottom we have used a length of the smallest available pvc pipe, notched out for the mesh.