A good fixture for reptisun 5.0?

I recently bought reptisun 5.0 24in only to find out its a long tube ive now found out it needs a fixture or something how am i supposed to put in on my 40l tank? Its just a big plastic tank with a lid like those storage boxes. Where can I get a fixture for the light. Dont say anything about the size of the tank because I will transfer my yellow belly slider when it gets older, right now its a 2in hatchling.