A PR can be made (DIY) with just the cone, basket, spder and added weight for tuning. Its even possible to omit the basket if cone has a larger surround and 2 are used back to back like this
https://www.spacehifi.com.au/image/...ep10-v2/earthquake-sound-minimep10-v2-d01.jpg
Use 2 10" passives on opposite sides, quite a bit of savings in real estate, all other things being equal
No need of calculation, first note the tuning frequency from the ported simulation. Now keep adding mass to PR till the tuning frequency is reached in impedance curve. The minimum impedance between the peaks is the tuning frequency.
http://images.audiojudgement.com/2016/08/impedance-bass-reflex.jpg