It's finished!

It's made from 33 pieces which bolt together with so many bolts that I had to leave some out because I don't have enough.

It's definitely nice and bright in there now, which is good because the sunlight is often so dark here that even though this aquarium is next to a window, it was getting more light during the night than during the day just because I've been awake at night and have the lights on. So this should help my plants to grow.

Here's the underside of it, after it sat on the aquarium all day collecting condensation and splashing from bubbles.

...and a close-up of the condensation...

...and a similar area with the peanut butter jar and the light bulb removed. You can see how the jar kept the condensation away from the electrical stuff, and also how I added two holes to vent the inside of the jar to the outside, so that if any moisture does get in there, it can evaporate out when the bulbs are later turned on and heat everything up.

This is the top side where the bulb holders and wiring go.

...and with the lid removed so you can see the wiring.

My only concern at this point is accidentally dropping the lid in the tank, and also the fact that 3D printed plastic is a bit porous and so it's not impossible for moisture to simply soak through it and reach the electrical wiring that way. Hopefully that's a slow enough process that turning it on every day will evaporate any water trying to make its way to the wiring.

I'm also not completely convinced that it won't melt due to bulb heat, but I've had it turned on for maybe 10 hours so far and I don't see any signs of that yet, so I don't so much expect it to happen anymore.

I believe I used about a whole spool of filament to make this. So the cost was $15 for the filament, perhaps $10 for the three bulb holders and electrical cord, and maybe another $10 for the light bulbs, for a total of $35. So it's kind of expensive, but at least it's exactly what I wanted.