I’ve done both. With prefab designs for companies: some are for specific woofers, some are just generic. When I do generic designs, there’s usually multiple boxes for one size of woofer with different airspaces, that way the customer can choose the closest box size to what their woofer needs. The designs I do for individuals, like from this site, they are completely done from scratch and designed around all those factors I mentioned. Most t/s come from the manufacturer; I’ve done it enough where I can usually spot t/s problems/mistakes from the manufacturer, like some Skar woofers.