The 5 foot length makes it harder without overdriveing the bulbs ,imo.But understand your not interested in doing that and I dont blame you.
I might be entirely wrong but, I dont think they make a 5 foot bulb thats less than 80 watts,whitch would be overdriven.
54 watter's only come in 4' lengths.
So the only option I can think of is using a combo of 2&3 lengths whitch have alot less output at 39 watts for the 3 footers and less for the 2's.
Im thinking these would not penetrate 24" very well.
Jmo though.
-Steve