Genuine Juwel tubes come in at 590mm,short of 24" and the wattages are lower than you require.
I have modified my hood of my Jewel rekord 70 and placed another tube in there.
See http/www.fishforums.net/index.php?showtopic=83620&hl=
Maybe you could do the same for more wattage. alternatively you could swap the modded conversion for a T5 "21"power compact (interpet)which come in various colour balances and are 55watts, the only drawback with then is they get quite hot.
I hope this was of some help
Regards
BigC
I dropped the 2 juwel tubes in my tank for 2 Hagen tubes (a powerglo and sunglo tube) and these are rated slightly higher at 20w (the standard in this length is 18w) - although only a slight increase there is an obvious improvement in the lighting at lower tank level and the fish colours are much more vibrant