Your thinking outside the box, that's good. Without it nobody learns. But your still thinking in the box, about a 6' wooden enclosure. Just make it modular, screws are cheap. It doesn't need to be wider at the most 30" but 24" works fine. Most newer homes have 32" doorways while older homes are 28". A T18 animal plastics is 24" wide an 60" long. It will fit any doorway, even a 1973 singlewide trailer, this I know first hand.
The problem with a grow tent is heating your reptile. With plants I use seed heating mats an grow lights. Grow lights produce little heat, most are LED now. Plus, everything is on racks an shelving. If you could regulate seed mats it might work. But heat lamps are a very bad idea. They will melt or burn the tent, just depends on the material. My tents are 10'x30' an I don't think they could keep any snake bigger then a hognose in it. An the hognose would find away out in time.
Think like a snake. Your in an enclosure that the sides move! You want out an you'll find it. Now, you found a vine (extension cord) an your going to climb it. There's this nice warm thing on the end of it an you want it. Now, unless the heat lamp has a cage around it your burnt. It could be done. But to mitigate all the risks it would be cheaper to buy a T20.
What ever you do stay safe an good luck!