Aluminum to skin smoker

A few things to consider.
Aluminum costs alot more than steel.
You will want to go thicker on the sheet aluminum than you would if you used sheet steel in most cases.
If you know what it takes to cut aluminum sheet. It can be easier to cut than steel. Otherwise aluminum can be a pain to cut.
Since you are doing this with insulation on the inside then the aluminum. You would want to have a very good fit on all inside edges/corners etc.

Since you would be doing very low temp smokes. I wouldn't think you would want to bother with insulation and all the work/cost. But your choice.
You could put together a much cheaper and easier to build wood smoking cabinet.
Just a thought.
Post some pics of your build and in action!

Could you not use cedar type planks on the outside of the Cabinet, timber has a reasonable amount of Insulation and it could look good.

I've smoked fish in Big Chiefs and Little Chiefs for 45 years, they are aluminum with a hot plate. The interior darkens up after a few cooks and there is no safety issues with the aluminum what so ever. I do like the idea of making the body out of wood, then lining it with aluminum.

Check out your local tin shops, they are the ones that typically sell the stainless sheets you see in restaurant kitchens on the walls or counter tops. They likely sell thin aluminum sheets as well. They also have the equipment to form corners or a pan for the bottom

You can use plywood, most any of the 4X8 sheet wood HD carries.
I would not use any treated lumber.
Lots of ways to do it.
Visit Cowgirl's site to see her cold smoker. Real nice
https://cowgirlscountry.blogspot.com/
This one is not mine, just for an idea.

Also if it is on any other metal you could induce Galvanic corrosion.