Well this is my design of clamping mechanism. Not such a good UX as those quick release levers but it does hold the tube very firmly and should work.
The angle iron is a bit bigger than it needs to be. So I overlapped it, drew a line, and cut slices off lengthways. Then each half of each of the halves if you see what I mean needs to be tacked with the inner tube in there, and also a 1mm or so shim because the welds pull in. When you take the shim out it's a nice smooth but tight fit.
The nuts on the back are "captive", i.e. welded on. So you just tighten the bolts from the top.
