If I am not wrong this is a 37,5 cm cube (50ish liters). It is a strange measure in cm but I think you have lots of options... depends on if you go high or low lights, and of course your budget.
Cheap suspended fixtures: T8 and PL (I think there is no T5HO smaller than 24w which is 60cm long). Easy for DIY and plenty of fixtures available. You can easily go from low to high light just adding more units (for example 2 PL clipped fixtures: you can chose 9, 13 or 18w)
LED tiles: TMC mini tile 400 suspended or mounted, other TMC tiles would be probably a bit overkill
Classy LED fixtures: ADA Aquasky 361 (if you just reduce a little bit), expensive (around 200€) but this size IMO is not that expensive compared to other LED fixtures or tiles, although not dimmeable (lots of light!). Several chinese copies available: such as Chihiro AquaSky or this one
http://socav.com/producto/pantallas-led-mas-was-30-cm/ which has the advantage of being adaptable in size up to 15 cm. But as mentioned, for higher lengths these copies are cheaper and maybe worth it, but for 30-36 cm, ADA AquaSky is on a very similar price range and at least you can go and shout to someone on the LFS if it doesn't work... unless you can speak chinese
More information would be necessary if you want more ideas
Jordi