Once you've got two of the fences and cleaned the drum up, trial fit the fences to the drum. Ideally they should just spring over it and stay in place. File them for a decent fit, and check that the height of the fence above the drum is consistent and the right height for the outside diameter of the tube it's intended for.

The two "scythes" have to be welded to the drum so that they are parallel and the width of the tube apart. Weld the first one next to the lip on the drum, starting with three evenly spaced welds about an inch long, then two more in between those, and so on. It's a good idea to preheat the drum before welding it, but if you do this in the oven take the oil seal out of the back of it or it will melt over the inside of the oven! This will not improve the flavour of the Sunday roast one little bit. As hot as your oven will go for an hour or two will help, or use the BBQ or a decent sized propane torch. If you can get it starting to turn blue then that's going to be about as hot as you need it.

Fig.4
Welding the first "scythe" to the hub

Once you've fully welded the first "scythe" on to the drum check that the tube size you're building it for will clear the weld bead, and dress down any high spots. Then use some spacers made from either the tube itself or the same size box section to position the second "scythe".

Fig.5
The second "scythe" positioned for welding

Weld this one in the same way as the first one going round the outside of it first. Then remove the clamps and spacers and run another bead round the inside of the groove where the tube is going to sit. Again check that the tube will sit in the groove without hitting the weld, it needs to be in contact with the outer face of the drum and both sides of the groove but clear of the welds.