theabortionatorI love big tubes honestly but if I were you i'd just build a shotgun rail for now with 2" pipes. IF you find a fatty tube somewhere in budget at some point grab it and get that rolling as well. Can't have too many rails.
Or go with the box if you want but a shotgun or single tube rail with the 2" with slide closer to the way a fatty tube does than a wide box.
You could kind of make a box style top that slide like a fatty tube if yout put some 2x4s together and had a middle at one height then slightly lowered the tubes to either side , and if you ran 2 more put them even more dramatically lower. You would have that feeling of the single tube slide on top but be able to lock on more from the side like you can on a fatty tube.
But def build something now if you can rather than way for a big tube. Get yur shred on
The main reason I'm trying to get something bigger is honestly because I'm pretty sketched out to rotate onto my 2' shotgun rails. It feels a lot easier to me to rotate onto a box or tube because if I don't make it all the way I can slide it out easily both that would probably work the same on a double barrel. My budget is like $100 or less.