What is MDO?
An MDF core with MAYBE an eigth inch overlay. I've seen the overlay be some REALLY nice cherry veneer, which would be the shiznit if you were going with a natural wood CP, and I've also seen it with laminate-looking stuff (it may have indeed been laminate, I didn't care much to investigate). It's a tad more expensive than MDF, but really easy to machine, flat as.....a board
, and nice to finish.
Menards is the only place I've seen it in my area, but right now, we've ONLY got Menard's and Home Depot. In a few months, we're getting a Lowes, and it's been several years since I had a Lowes to go into.
Go with plywood. IMO, you'll find the walnut easier to attach to the ply than the particle board. If you still want to go with that, something that MAY make the particle board machine somewhat better is to get some generic masking tape and tape BOTH sides of the cut, if possible. Particle board in and of itself isn't a bad material to use for a cab, it just "flakes" so much when machining it, your edges won't be as crisp and nice as I'm thinking you want them to be.
I've been contemplating picking up some edge-banding router bits for just this purpose with plywood. I'll see if I can find 'em and link 'em, otherwise, you could do a nice little dovetail or t-joint or something equally esoteric that'll have us "ooh-ing" and "ahh-ing" at your madd skeelz