One more trick.
One of the pictures above is a close up of a white nylon bolt that I use to fasten the acrylic top down to the black plastic rim of the tank. It might be hard to tell from the picture, but the bold actually slides through a 3/4" length of round acrylic tube that is glued in between the two flat acrylic pieces that make up the top. I did that because I didn't want the acrylic to bow from the pressure of the bold being tightened, and because I wanted the airspace inside the lid to be completely sealed (not let water from below get up inside the lid). Given that the bold is so close to the edge, I probably didn't need to go to all that trouble, and could probably have left out the acrylic tube. It still would have been good to make a little rubber seal/gasket for bold hold on the bottom of the lid, to ensure that water couldn't get up inside.
I should also point out that this picture illustrates one of two ways I attached the bolts to the black plastic rim of the tank. In most cases, the rim had a flat band about 1/2" wide that the top could lay on, and I simply drilled a hole in that flat band and used a nylon nut and bolt to bold the top to the rim. However, in one place, the flat band on the rim had been cut away (manufacturing flaw?) so I had to get clever. As you can see in the picture (sort of) I made a little "L" shaped piece of acrylic that was long enough to slip under the lip, inside the tank, and stick out about 1/2". Then I drilled a hole in the "L" shaped tab that I had made. I could have used a nut behind the tab, but I didn't want the end of the bold to be long enough to touch the surface of the water, so I used a tap and cut threads into the hole I made in the acrylic tab, so I could thread the bold directly into the tab and tighten it up. I wanted to mention that trick, in case anyone finds themselves in a similar position.