Welcome to the community.
That’s going to be a really fun project to make and a very cool piece of art to hang on the wall when you’re done. If you used a resin and filled in all of the voids in between the tins you could make it into a really cool table top.
Can I say that’s a lot of Altoids tins. It must have taken you a long time to gather that many.
I love when people up cycle things that would normally end up in the trash. I’ve used a few Altoids tins for small parts in my tool box but that amount is crazy.
OK so let’s get started.
I would use ½” plywood for the back. Make sure you get a good sheet of plywood so it won’t warp on you. The cheaper plywood normally warps. I normally use birch plywood for my project since it tends to stay really straight. It’s a little more money sometimes but when I do a project that I want to stay nice it’s worth it.
The outer framing I would use a solid wood like pine or alder. It normally looks better if you attach the outer trim to the edges instead of setting them on top. When you set them on top you can see the edge of the plywood. All you need to do is cut to size and then glue and nail them to the plywood. If you cut the corners at a 45 degree angle you will not see any of the end grain so will look better.
Water base paint is a lot easier to clean so I normally use that. For projects like yours it works great. Make sure that the paint has cured completely before you start to glue the tins on. If the paint is not completely dried the adhesive will not adhere properly.
Here is a link to a clear paint that I think would work out well for you.
Don’t worry about the paint or glue warping the wood it will work out great.
It sounds like you have it figured out pretty well already. I did add in a couple of woodworking hints that will help you along the way and make the final project look a bit better. Other than that I think you’re on the right track.
Please post some pictures of your project once you’re done. I would LOVE to see it.
Thanks again for joining the how-to community.
I can’t wait to see it!