You definitely want the instruments inset quite a bit if they're going to be in the seat back. You could make a box or take a look at SSI's coaming boxes
. One of them might be a good size for your instruments.
Our boat came with a radio mounted in the cockpit coaming. It didn't last the season with us, and we tried to be careful. It stuck out a bit, so it was kicked and kneed and just generally unintentionally abused. (For now we've gone to a handheld unit at the helm, there's another installed at the nav station.) Here's an old pic that shows the radio. I've since built a pedestal table for the instruments at the helm.