they recommend 5-8cuft so in between there u will be fine and like cloak559 said 33-36hz tuning
i would go 6cuft tuned to 35hz with about 1500-1700 wrms if you can
i have a 2007 Punch p3 15 in my galant and havent had a problem with it yet have 750wrms going to it and it says its 500wrms handles it great
i beat the hell out of it and never had one single problem
i would go atleat 2.5cuft and calculating is the easy part its plenty of people here that will help you designs a box and make cuts sheets to where all you have to do is cut and put together
yea i had my 15" p3 in a prefab and i couldnt stand it i built a 3.5cuft enclosure and it pounded twice as hard
defiantely try to build one sooner you will be alot happier
23 years is a little over the top may like 5 years would have been alot better just think of it as your retirement maybe add little by little to it and fall bakc on that when you retire 20years from now