this is what I did for my thirty gallon with a similar top
just get a sheet of plywood, cut it to shape and then cut a hole in the middle for a light but make sure you make the hole a bit smaller than the light, in both length and width. that way the light has no chance of falling in. lastly you cut a fair sized length of plywood off the piece you already cut to fit the top of your tank and attach it Back on with hinges, if you wanted it to stay on better, just get those small magnets with the sticky Bottom and put them around the edges, that way you can take it off but it still stays stable when its on
best of luck!!!