sounds like a good time to head to a junkyard and get a windhield washer reservoir off something- i don't know how big it needs to be, but the washer tank on early 70's Novas wouldn't look out of place, and it has a seperate steel mounting bracket so you don't have to take any bolts off to remove the tank.
the washer reservoir on my 74 Monte could also be fit somewhere under the hood of a g body, but it is more of an awkward shape.