go for threading in a pipe plug.. the whole thing could be done for less than 5 bucks, probably less than 1.50 if you found someone to loan you a tap and drill bit. A bolt isn't going to seal because it has straight threads, the pipe threads are tapered so they jam in.
it will probably fit 1/8" NPT, harbor freight should have the tap, or an autoparts store, or anyone really. drill the hole through and jam it in with some teflon paste or loctite.
i've also drilled out the holes and put in aluminum plugs and welded those in. You cant just straight weld the hole in, but a little plug and clean surfaces and you can TIG it right in there, or pay someone to do it for $5
pro tip, drill the hole to fit whatever aluminum you can find cheap and easy. 3/8" probably ?