I think they can be a great idea. If this wasn't my flat like fook i'd have cut holes in the ceiling and walls
I think it should be pretty easy to make one from scratch, just a question of making sure water temps won't exceed the pumps safe running specs, insulate lots
and make sure that all of the water around the bulb is being circulated, not just say the top 5" of water. Would be easy but not the cheapest. Something i've not read up on though, how do the lights typically cool down the water? is it through a chiller or just effective radiator? I mean if your flow rate is high enough, and your water capicty large enough, it stands to reason that you can get it running without much heat buildup in the water, and as such could get away with an old car radiator etc
All depends on what you can get your hands on really. I tend to make plans, and then source specific parts and as such end up paying more than buying retail