The thing about this is, you are just making a band-aid! It may last forever.....but.....the fact of the matter is that one day it will likely burn up. Underground splices with AL are hard to get watertight.....even with heat shrink and everything else available.
My advice, make CERTAIN you know exactly where the splice was made, so if it burns up, you know where to fix it. I understand about budgets.....but do what you can to make for a easier fix when it does go south.
Use de-ox or no-lox gel/compound when making your splices. Use it liberally, once your splice is made, cover the entire connector in it before you put the first heat shrink on. I would recommend no less than 3 heat shrinks, getting progressively longer by 2' increments. that way each heat shrink is 1' longer on each end than the last.
remember.....more is better here.