This is my 4ft tank UV light mounting position.

Here is the steps & pictures

1) Cut a slot on both sides of the cover tray. U can skip this process if you dont intend to cover the OHF tray.

2) Source for connector fitting that can fits into yr UV connector. Ensure that the outlet connector also can fit into yr rain bar piping. I used the 3/4" tubing and fitting where you can get it from the LFS shop or 13mm grey color piping where most hardware shop has it.

3) Rain bar piping DIY - i used the 3/4" tubing, drill hole onto it, then hook up everything and you are good to go.

Observe that flow of the entire system, make sure that the "flow" is smooth, no sign of water clogging or leaking etc. Also, if you observe carefully, after insert both ends of the fitting to the piping. The UV can stays in a straight position without support bracket at the bottom.