No website for the info but really pretty simple to build with a few tools. What I had in mind was a one inch thick board of whatever width you want the shelf. Maybe a 1X6 for betta jars? To keep it from bowing down in the middle if it is not supported, run a smaller board like 1X3 placed on edge. To make it as strong as possible, I like to use glue like Elmer's carpenter glue and then a few nails or screws to hold it while it dries. This board doesn't have to be very wide top to bottom. Being on edge, it is nearly impossible to bend it even as thin as 1X2. Looking at the end, you might want to see something that looks like an "H" but with short legs? This gives you an upper edge to hold things from sliding off and a bottom edge to hold the shelf on the tank top. If you make it just slightly longer than the tank and cut a small notch at the end so the leg of the "H" can drop down inside the lip of the tank, it can't move unless you really, hit it hard enough to lift it.
What you will have is a wooden version of an "I" beam. Very strong with minimum weight.