I had the same idea, but there are problems with using the top half of a PCE as the monitor back; you'd ideally need to do something with the Hucard slot, i.e. move it to where the expansion port currently is, and then cut out and fill where the slot was to regain that space.
The way I'm getting around that, is (once I get my 5.6" screen), I'm going to take a mold of a PC Engine in non-shrink clay, but with modifications (deeper, no Hucard slot), then make a plaster of paris casting, and use that to vacuum form.
I'm hopefully (eventually) going to use an AV booster to hold the hinge attached to the screen, and to hold the battery and speakers, so your mileage may vary, but if I can successfully cast one such casing, two wouldn't be a problem. Mine are gonna be about 2mm thick plastic by my reckoning, though I plan to try a few things. I'll post something up when I've done it, which should be next week - I have the screen on order (from China), and the clay, but I still need to get some MDF and a drill to make the vacuum forming table. And the plastic.