Quality welding of aluminium requires special equipment and skills. In case you care, aluminium will oxidise when molten, ruining weld strength, unless it is kept out of contact with air. This is done by a process called MIG (Masked by Inert Gas) in most countries, and Heliarc in the US. The inert gas used is normally argon everywhere but the US, and helium in the US, hence the different name. High pressure gas from a gas cylinder is blown onto the welding job from a discharge ring around the welding contact. Sometimes the filler metal is fed automatically through the contact in the form of wire, and sometimes the operator holds a rod of the metal in his/her other hand. So, not very many people are equipped or skilled to do this kind of work, and those who are can charge high prices.
I agree using thick aluminium plate for the repair, rather than steel plate, is the best way to proceed. Steel in contact with aluminium will cause the aluminium to corrode when wet (the aluminium acts as a "sacrificial anode"). Hence to make this a durable repair you not only need to repair it with aluminium plate, you also need to use aluminium rivets. There is still a problem with the stump of the steel expander inside the aluminium pop rivets (part of the expander remains in the rivet after most of it breaks off) and ideally you would drive each of these stumps out with a pin punch, but hardly anyone does, except on aircraft repairs.
