I just use a small bolt through the kidney (from the back) and into the cod and use a stud (like the shoulder stud) to keep it closed. They need to overlap to be able to do this. I can post a pic if that wasn't very clear.
Depends which armor you have. I have a 36: waist with FP armor and i didnt need to shim it. Some makers the kidney plate may be shorter so it just depends on the make. Just hold it up to yourself and see!
Bounty 1138...Shimming is adding extra sheet plastic to one of the parts to make it bigger
I also just use the studs on either side of the cod/kidney to hold it up. Also have Bobamaker armour and there's quite a lot of overlap for me and with the weight of everything sometimes sags a bit in the back so I might end up installing some snaps or velcro to secure it to the flightsuit.
I've got RKD armor and it goes around me fine with a few inches to spare. Basically, mine is all velcro on the cod (hook) and attaches to the kidney (loop). Studs are just decorative.
Additionally, I thought I was going to have to add some velcro to keep it up and for stability. But it pretty much stays up on its on.