Every crossover needs to be custom tailored to the set of drivers and enclosure it’s going into. In order to get the best results out of any speaker system, this needs to be tuned in every design. And off the shelf crossover like the one you linked will function but it won’t perform optimally. You should definitely buy the crossover plans for the kit that were created by the expert designer any chance that you have.
Grs 12SW-4HE is a surprisingly capable subwoofer for $60 on parts express. Check it out and model it in your box to see how it will do.