Hi Paul, Sorry to bear the bad news, but it's not possible to solder aluminium, even welding it is a specialist thing, I think it's to do with it oxidising at relatively low temperatures i.e. heat it and before it gets to molten you get lots of oxides forming that stop you making a clean joint, so it has to be welded in a 'protective atmosphere' of inert gases.
Anyway, if joining alloy parts I would use an epoxy resin glue, make sure the fit of parts is spot-on first, then hold them in place with whatever; clamps, weights, blu-tac et al, then add a fast setting epoxy and warm it with a hair dryer to encourage it to flow and fill joints. I'm sure others will also add their advice. Make sure you post some pictures for us.
Have fun
Liddy