Purchased the Kit through Fleebay seller. Overpriced Kit included three elements with pigtails, a totally useless oversized foam sponge into which the electret elements were intended to be inset, and a voltage regulator.
Certainly the electronically inclined could accomplish the assembly of parts @ <1/4 of Fleebay cost.

Problem was the oversized nature of the sponge made impossible alignment of the elements straight forward. it was simply impossible to stuff that mass of material between the faceplate and the backside. Was like trying to bury a politician in a shoe box; which can be done, minus the BS.

Made my own 3 electret element mount using 2 7/8"D x 1/4" ABS disk; and, made allowances for front face and backplate attachment screws, around the periphery of the disk. Mounted elements in triangle pattern points 20mm from center and ~ 32mm apart. Rock solid fit with wiring fitting behind disk and sandwiched between thin felt and backplate.

Reportedly sounds "good".