My solution for a cheap combiner box - roof mounted

I just use a 6" square PVC box, the small ground bus bars and waterproof fittings to bring 4 panel feeds in. I have the leads fused in the box and my 6ga feed down to the controller comes out of the bottom. I've used the same setup on 2 class A's and never had a problem. Cost less than $20 for everything.

Do whatever works for your setup. Higher price doesn't mean better.