Nice job on the website, and I like the simplicity of your cage top design. I especially like your lockdown method of a nylon screw in a tapped hole. I'm gonna use that idea.
I make my aquarium toppers somewhat similar.. No pics yet but coming..
1. Start with the top frame of 1x2s, wide side down like a picture frame (the higher-quality straight pine) that sits on top of the aquarium trim and extends out about 1/16" on each side of the trim past the widest point of the trim (the lowest part of the aquarium trim sticks out farther than the top). I just glue these together with no nails, clamping them tightly for an hour or so. Use a power miter saw for the cuts and good wood glue and there is no problem.
For a 10 gal, the pieces are 20 5/16 and 7 15/32, 2 of each.
2. I got pine stop molding that is about 1/2" thick and about 1 3/4" high and cut 4 pieces to go around the frame to hang down over the aquarium lip. I glue and clamp them in place. These pieces also help strengthen the original frame.
Now you have an open frame that sits nicely on the aquarium top. From here one can cover with a pane of acrylic or get more fancy-shmancy. One of mine has a hinged door (wood frame and acrylic window.) Another has an acrylic window for most of the top then a plywood section that supports an incandescent light fixture mounted on a 4" dia $1.99 plastic pipe cap from Home Depot.
My opinion, the more one uses a frame and acrylic (or expanded PCV) for these tops, the less wood exposed to humidity. I use to make tops from plywood, but the framing method with acrylic window means less effort sealing wood, less odor in the tank and less offgass time. Plus, it is easier for me to use a miter saw to cut 1x2's with repeatable precision than it is to cut plywood with repeatable precision (I don't have a table saw.)
I made a similar top for a big Cambro poly box using 1x3's instead of 1x2's, and it rocks. I'll be using your nylon screw idea there for sure. I'll need to attach a piece of wood widen the molding at those points. Man that screw method is a great idea. I never thought of that in all this time. I was using hooks and such before, yes even a book or two. What a dolt I am. Thanks for the tips.
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Mark