I am doing my Tech IV right now. My baby daughter is playing with the old tan material, no doubt full of carcinogenic adhesives.
I just got the cheapest material, it looks very well cut. It seems to stick fine even though I haven't removed all the old adhesive. I do need something to remove the gunkier, stickier stuff, rubbing alcohol isn't doing the trick.
Linhof uses some thing metal foil and some filler in the recesses under the covering. I am not going to worry about it, as this is a beater caemra without the rangefinder or viewfinder gizmos - I just covered the holes up with an extra layer of tape, which shows through but doesn't look bad. Nobody will mistake this for a Technika 2000 though.