Is that before or after wood displacement? You can fit 1 18 no problem, 2 is going to be a tight fit especially if you were looking to do a lot of port area or tune low.
What are your dimensions? Subtract 1.5" from each dimension to get the gross inside volume. I would subtract 2.25" from the height because I am sure you want to do a double baffle (or if it's a wall, do the depth or something, doesn't really matter)
2 18's in a trunk will be hard I assure you, its difficult for me putting one 18 in my trunk. Mainly because of height, rounded edges entering the trunk etc..
I mean through my trunks dimensions before any displacement I have 10 cubes to work with, but even just throwing 6.5 cubes in there was the tightest I could fit.