I am also recovering the frames for the bass bins. THIS has been an eye opening experience for sure. I have yet to fully discover how Martin Logan got those frames covered so perfectly but I am getting there. The first frame is almost done and it has been my BETA test.
There are some things I can say for sure.
Standard speaker grill cloth will not work! I bought 4 yards of the best Mellotone from Parts Express. Wasted money. It does not stretch. Prodigy frames are just odd and crazy deep at the bottom half of the frames. Matin Logan original cloth may be some type of Lycra but it stretches in one direction a lot. Not so much on the other axis. I found some material at
JoAnn Fabric (going out of business

) and bought 3 yards of it for roughly $50. It stretches in both X and Y axis. One yard should do 2 frames. I bought an extra yard in case of mistakes.
Martin Logan used glue to put the cloths on then dressed up the inside edges with a fabric ribbon, maybe to keep the edges from fraying. I am doing something similar. There is a popular speaker glue that is rather expensive for not much glue. I have found that Yeardera fabric glue is essentially the same thing and you get twice as much glue for less money. It smells exactly the same. Basically, contact cement. I used a black Outus 32mm (3/4") wire harness cloth electrical tape from Amazon that works perfectly when glued a day after the grill cloth has been glued for extra edge protection.