Your best bet is to contact a local cabinet maker. I'm sure if you are willing to pay them they might even assemble the entire thing. Your other option is a car audio place. Just make sure you tell them you want the cabinet built with 3/4" birch plywood. And you will probably need to give the cloth wrapped rock wool to them.
The materials for wood for 2 of these cost about 150. So one would be cost around 80 for the cabinet materials. If you want a simple DIY option I think Dayton has kits. You could easily make those better with a couple of mods.
It can be done. FYI you can get home depot to rip the plywood and then you will just need to router the whole and glue the materials together. It really is a simple build in my mind if you have the right tools. You can even get a jig to mount on your router to cut a perfect hole.