I have a 20 gpu's being mostly 1070ti's and some 1070's in a longer single car garage. I also use it as a work space and man it gets hot in there. Even in the winter. I am planning on making some kind of a mining rig box but I am in the early planning stages. I was thinking a 2x4 frame with drywall walls?
You gotta be really careful when designing so-called "mining boxes". The temperatures can really get out of hand when not enough air flow is given to the box where you have your rigs in. Can really become a serious fire hazard.