If you really want to use Aluminium, the cheapest way by far is to buy some Aluminium bar stock, place it in your lathe, drill, tap and part off as required. They really are very simple for sure and it will only take you about 30 minutes to an hour from start to finish to make the whole set and prepare for the next job for the Lathe.

If you don't have a lathe, either pay the price for someone to do it (maybe you know someone with one who will do it for you for free or for a small "gift" - maybe you will have to pay a machine shop to do it)

Alternatively, get some hardwood stock (I use 3/4" square, some use 5/8 or 3/4 round) cut it to length with a mitre box (to make sure it is cut square) and drill through the centre with a drill (shouldn't need a long series for 2" long) and use bolts right through them.

Even though I have all of the equipment and material to make them efficiently, I rarely use long aluminium standoffs nowadays. I much prefer the wood ones, usually with a mounting plate directly behind the engine.

The setup shown below weighs nearly an ounce less than 4 Aluminium standoffs of the correct length to do the same job would have weighed.