My tank was odd shaped, and frankly, the commercial ones are not that tough (or attractive) so make your own and fit it to your tank, and your house!
First off, measure the tank. You want too much material as you are going to need a bit more than the measurement of the tank!
My Tank is 16x48 1/2 inches, so I got the 3x5 roll of screen. The type I decided on for strength and looks was a woven mesh square cut, often marketed as 'wire cloth'. It bends and cut easy but is strong enough to have two cats standing on it!
For the wood, I chose Hemlock, in a 1x2 size. Since the tank is 48 1/2" long and 16" wide I knew at the exact measurement I would need 10.75 ft of wood, but got two 6 ft lengths to make sure we had the extra that we need to make sure it fits right!
First thing you want to do is unroll the wire cloth over the tank, it will be stiff and want to roll up so if you need, someone can hold down an end. I did this myself so it can be done! Bend one end down, flush with the tank. Go and do the other end.
Trim it to about the depth that you want it to be, make sure it is long enough for the wood you chose! If you look at this picture you will see I left plenty of room to attach the wood to the screen.
You want to leave it a bit long on the sides as it will stabilize the top by being able to fold these over the flaps that you are about to create, so you want the sides to be about 1/3 longer than the trimmed edges.
Next, you want to cut the screen to the inside corner edge and bend down the sides. Do this all the way around. Fold these flaps in.
Next, fold the excess screen on the sides up, and over the flaps.
Now it should be nice and stable. Almost strong enough to leave like that, but it would be not only hideous but rather painful (all the exposed cut edges...if you leave it like this, get a tetanus shot!). Time to head outside or to the shop!
Sawhorses are great, but if you just have a table, that will work fine as well. Cut the wood to be 1 inch LONGER than you measured, so if it is 48 1/2", like mine, it needs to be 49 1/2". If 16", the 17", and so on.
If you have a heavy duty staple gun, staple one piece at a time, keeping them even (upholstery tacks are a good alternative). Make sure to place the tacks or staples about mid section on the inside of the screen. Check the picture to see where I put the tacks. After you have the wood tacked or staples into place, use finishing nails to tack the wood together and you are done!