well if you dont have accurate t/s parameters its not really worth it to pay for or have someone design something just use the 10-15 sqin of port per cubic foot guideline and make it fit as best as you can.... it should work fine for most setups but you really loose out if its not designed around actual specs.
measured specs are the only way to go unless you are going with a very well used well known woofer a half a cube here or there and a few inches of port length can make big differences between 2 woofers things like cabin boundaries and port clearances are very important to take into account too
modeling 10 different 12" woofers in the same box can give very different results with each one then potting that box in a car can totally throw off the results if the port or woofer is near a boundary or wall or seat or one is near something and one is not....
good luck