The total performance of a loudspeaker system is the sum of its parts, so just copying the cabinet is not guaranteed to give you that performance you heard before. The driver matters too, as does any upstream processing/EQ being done as part of the integration of the sub with the rest of the system.
Unless you can replicate all of that, I wouldn't be too bothered about trying to copy one exact design. The "best" cabinet in the world can still sound rubbish with a mis-matched driver, or poor processing.
If you really want the exact sound you heard before, buying that product is the best way of getting it.
At least Carvin seem to publish TS parameters for their drivers so you can get a rough idea of what to look for in any potential replacement candidate. https://carvinaudio.com/collections/speaker-parts/products/ts18b-8-800w-18-inch-woofer
As for the port length, do you know anyone who still has these boxes? Asking them to take a grille off and measure for you would be the easiest option, if so.
Failing that, see if you can get a used one for cheap on EBay/Craigslist etc, measure what you want yourself, then sell it on again if you still prefer to manufacture new boxes yourself.
HTH,
David
