Thin aluminum is easy to cut (think gutters). What you'll probably run into is that getting a very clean, straight edge is going to be next to impossible with tin snips. This will require a large shear. you will need this smooth edge in order to get an adequate seal.
Another thing, you will want to use teflon gaskets most likely. Rubber at high temps isn't really your friend. Silicone wold work, but in order to get an adequate seal over such a large surface (unless your plates are VERY flat) you will want something that won't squeeze out under compression, and I suspect that silicone would do just that, but I may be wrong.
Also, you will want to use a very heavy gauge metal for the front and back plates I suspect, or else use some angle stock around the edges that is very flat. Even if you put a lot of bolts in it getting even clamping pressure on your gaskets is going to be difficult without eliminating as much flex as possible from these plates.
As far as the fluid connections, I would look into soldering them onto the faces of the plates with silver solder. Look up the keg tool, then use that type of solder joint with a 1/2" coupler and screw the appropriate fitting into it.
Finally, convoluting the plates so they all match and still have nice flat edges will be tricky. If they don't match you run the risk of pinching them, and once again without the flat edges clamping will be tough.
I'm not trying to discourage you, and sorry if I sounded like a negative Nancy. I'm behind you 100% on this, so keep the ideas flowing, and keep me posted on how it proceeds. I think that this is a great idea, as long as you can source the materials cheap enough.