Aluminum shrinks .006 per. in. from 1200deg. (molten) to room temp.
I did some lost wax casting in high school, in bronze.
It is not hard to do, I would start with a mold made from an AR-15 lower made from rubber or latex so it will stretch just a little for shrinkage. Then use it to make a wax form, attach round wax called sprews to it, then mix & pour casting plaster (it looks like plaster of Paris) over it to form your mold let it dry, set it in a kiln or oven to melt out the wax & heat and temper the mold.
Get your aluminum molten you could just pour the molten aluminum into the mold, but if you want to make sure it fills the mold with no gaps I would look into buying or building a centrifugal caster. The one I used in high school had a spring that you wound up & when you were ready to cast you pulled it back the stop pin dropped out of the way, then you let it go to start it spinning. And the centrifugal force of the molten metal will press into your mold & fill all the gaps. When it stops spinning the mold will still be hot, but your metal will be set. You take the mold then dunk it in a bucket of water, & the casting plaster will crack and fall off. The hard part is digging out the plaster from the inside of the casting, a pressure washer helps here.