DIY coolant overflow tank?

[dmention7]

So, Pete's post about the DIY catch can (which I started building today) got me think about replacing my factory coolant reservoir. Right now the stock one is kind of hanging out in a weird spot, and it's in the way of not only my intake, but also the access to the headlight bulbs. I'd like to replace it with a simple canister that can be more easily positioned to a convenient spot, and that looks less ghetto.

What I'm a little unsure of is aside from acting as an overflow reservoir, does the tank have to hold pressure? The stock reservoir has a little vent tube coming out of the cap, but it's my understanding that cap has a crude valve to maintain pressure in the cooling system... in which case any replacement would also have to have a similar valve... or am I mistaken? Google wasn't helping me out much.

(and yes I know I can just buy what I'm looking for, but it's not in my nature to buy what I can easily build)

EDIT: a little more searching, and I think I was mistaken... it's the radiator cap, not the reservoir cap, that contains a valve to release coolant to the reservoir once the cooling system heats up and builds pressure, and to draw coolant from the reservoir as the coolant cools and contracts.