OK, I kind of have a plan now...

I bought these roughly light bulb-sized jars of peanut butter. They're actually a bit smaller than the 100-watt equivalent bulbs I was planning to use, so I bought some 75-watt equivalent bulbs which are just small enough to fit. I figure I can make up for that by using 3 bulbs, which is actually a better plan as I'll then be able to symmetrically light the aquarium with 1, 2, or 3 bulbs, giving me a choice of brightness.

I emptied all of the peanut butter into a container and gave it to my mother. Amusingly, this is far cheaper than just buying containers. These things were $1.18 each, whereas buying empty containers would have cost $5 to $10 and I wouldn't have had as wide a selection to choose from.

So I just need to work these into the design so that I can screw the jars around the bulbs and thus keep the humid air from condensing onto the bulbs or bulb holders.