I have never tried the "brazing" type wonder fix rods but they might be ok? As above, watch some videos and see what you think. You could just buy enough parts to make one pipe and try it. You must have a mate you can borrow a blow torch off? Do you already have a drill and a tap/tap holder for making the holes in the ends? If you make it in alloy you will either need something like a cold cut saw to cut the plates or some way of cleaning all the cuts up after cutting as aluminium welding is very sensitive to dirt so any specs of paint from a hacksaw blade or dust from a grinder disc need to be completely removed before welding. If you had a working model you could the rest made up for you. If you cut and drilled/tapped all the end plates yourself there are a few guys on here who would give you a price to weld them onto the ends of the box for you. Might work out cheaper as just buying a gas bottle can get expensive. I would expect you will be looking at at least £1000 for a used ac welder with the other bits you need, probably more. So that is going to put a decent dent into the profit margin although if it's a one-off project you could sell on all the welding kit afterwards and make most of that cost back but you might have to spend a bit more in the first place on some branded kit if you want it to sell again easily. Personally, I would make the whole thing from round copper tube as it will be easy to connect pipes to the end using easy to find fittings. If you need flat mounting surfaces either get some copper sheet and cut a square the size you need then a load of smaller steps in rectangles to make a saddle you can clamp to the tubes with some thermal paste filling in the gaps or find some square copper bar, get a radius cut into one side and solder it in, or even cut into 2/3 sides and sandwich inbetween pipes. Another option would be to buy some big solid lumps of copper or aluminium and drill a load of holes through it, tap the holes each end for fittings. You will have to loop a few of the holes together with pipe unless you get creative with cross holes and blanking plugs but that way you can adjust the amount of cooing by drilling more holes if you need them. Copper sheet could be soldered up into boxes, you can buy all the stuff you need for that from screwfix/any hardware store. I can imagine it will be quite tricky to solder all the way around without melting the bits that have already been done and you might want some sort of jig to hold it all together. You could make it easier buy having the sheet folded into a u section to give you a start. Or you could buy a dc tig and weld the copper sheet that way. Another simple option would be to get a block of wood, drill through it with the same size but as the tube, cut it in half to give you a half-round former. Take the copper pipe, fill it with water, freeze it, then lay it in the half round and beat the middle of the exposed section down flat with a hammer. Then either clamp a bit of copper sheet on with some thermal paste or solder it on. Or copper tube, cut a section out of it and solder flat sheet in there. There are hundreds of ways of doing it without having to buy the kit. If you think the way you have planned will work then post up a rough drawing with the sizes on and somebody can give you a price to make it up or you will get some suggestions of ready made stuff that might be a suitable replacement .