I was pricing out plastic material at mcmaster carr, and by the time you start adding in shipping it starts to get quite pricey. If your project doesn't require perfect uniform thickness or a specific material A good place to check is your local discount store.
Polypropylene cutting boards are cheap, mostly uniform, flat and available in your local town. I got this one from walmart for 9.99 + tax and instantly put it to work.
Ordering the same material in a similar size would have cost 15 bucks for the stock + 10 bucks for the shipping. Even though its a tad over a quarter inch thick (my ideal) , and its not 100% perfect, it doesn't have to be.
Poly is also extremely easy to cut, drill, tap, and machine while still being really tough, the bad side though, is its nearly impossible to glue. There's some off the shelf products that can do the job, but more on that later, first I need cut parts to eventually glue and more importantly, screw together.
While I have made rough plans and had a great idea of what I wanted, I have never sat down to figure out the exact details. Armed with a ruler, pair of calipers, a sharpie marker, and my trusty TI-1000 calculator I sat down over the last couple evenings and figured it all out.
Next is to cut the parts and mill the dado's, I think I will do the dado slots first while its one big easily squared up panel on the end mill at my work (course you could use saws or a router as well)