I've used 3M 77 spray adhesive, starting with a good degrease/rough up of the holster prior, as mog mentions above.
I have used both 330 and 500 denier with similar results, but the 500d cuts easier with a hotknife. The factory-coated Safariland holsters I have look like 500d to me.
After applying the slightly-oversized (at the edges) fabric, I vac-sealed the holster for a few hours to get the fabric as tight as I could to the holster body.
Afterwards I would remove the holster from the vac-seal bag, let it air cure for a day or so, and then take a hotknife to the edges.
I'm not sure the vac-seal part is necessary, but it has held up well on the half dozen or so holsters I've done for others and myself.