If you do a search you will see a lot of people built there own. Some of them used a small tool box from a pickup. Depends on how much room you have on the front of your TT. Another thing to think about is that some of the members here live in a high crime area and have built tough battery boxes to deter thieves.
I just used 2 cheap, plastic, boxes. They are made for smaller 12 volt batteries so the cover doesn't fit on there perfectly but, it fits over the sides good enough and then I bought 2 cheap, short , ratcheting straps to hold the covers on. Good enough for me in my area but, it is not at all theft proof.