2 ways, make a frame of angle iron, "U" bolted to draw bar or you could use long cup head bolts thru frame to bolt with 2 hole fish plate to draw bar and bolt the box to that, this will give great support and save the floor of box cracking up later on or you could mount box directly to draw bar using "U" bolts, 2 each side with a plate inside box floor to spread load of bolts, holes slightly larger than bolts, ideally with some neoprean between bolts/box and draw bar, allows just enough movement for flexing, nuts need to be nyloc or double nuts to lock............... there will be some flelxing between the bar and the box..... I would go the frame for the box then you won't find in time the floor cracking and coming away, especially if you intend to chuck 35kgs of gas bottles in it.
Welding on draw bar is a No, No, can cause a weak point if not done correctly and not every welder has the nouse........ so only brackets.......... TEK screws certainly Not they would shear off very quickly and also weaken the draw bar structure......