Well it's done. Gaps are bigger than I wanted, I think I needed to make the dowel holes a bit deeper. But with glue filling them, sanding the transitions, and adding some paint / decals it should look fine. Mounted the speakers tilted down so the sound hits you. I did have to add 4 screws to help stiffen up the joints a bit. 2 dowels per joint was just not sufficient. I still need to drill the mounting holes that line up with the cabinet threads, and add a cutout in the center to route cables through.
Also got the siderails done. Was going to use the bandsaw, but a co-worker told me maintenance has a shear for straight cuts. It was only 36" wide (which makes it hard to cut since my piece was 36" plus I'm cutting at an angle), but after splitting up a 12" wide piece it fit just fine.
Basically I'm doing this to not only not worry about my gaps between playfield and cabinet, but I can add sideart without worrying that I'm going to scratch it (it's part of the playfield). And bonus, in theory, I could play a game completely outside the cabinet since it will be completely enclosed.