First many Spa/hot manufactures will require #6 copper as a minimum wire size, and some will even require it to be on a 60 amp breaker which #6 aluminum will not be rated for, so reading the manufacture installation instructions will be the first step, if all the above is ok then I would say there are many UL listed aluminum to copper splice kits on the market, some pricey and some no too bad, but it must be listed for splicing aluminum to copper, I use the Ideal DB 4-wire kits as they have 4 terminals, with a heat shrink to cover the splice, which also has a hot melt glue inside it that melts and seals the splice, Ideal also makes a wire nut called a twister listed for aluminum to copper, but not sure if they go this big?
But read the instructions first, as I wouldn't want to be answering to the owner if you void the manufactures warranty.