A different choice of raw material may make your project easier to complete. Aluminium is difficult to solder or weld without specialist materials or equipment, but seams in brass, copper or mild steel can easily be closed by soft or hard soldering.
Folded seams may be possible in aluminium, or make a butt joint reinforced with a rivetted strap.
A relatively short length of tube, eg the length is less than it's diameter, can be roughly formed by hand methods , using little more than a good stout forrmer (an offcut of scaffold tube or 2" iron water pipe) and a mallet, then true it up to make it as round as you like after finishing the seam.
Fixing the base could be more problematic part, unless you could use a wooden disc simply pinned (nailed?) in place.
Model engineers' suppliers are often able to supply short pieces of tube in a variety of sizes.
Have a paw through the selection of 'big bore exhaust pipe trims' sold by Halfords and the like to see if one of those can match the size you need, or try anywhere else that sells 'round metal things' - I used a small camping billy can bought from the pound shop as a magneto cover for an engine I was building - it was exactly the diameter I needed, and cheap!
