What I've done with a couple of my cases is use a spray adhesive as well as the 'sticky back' of the decal then when all the stickers were on I coated over it with a matt varnish spray.
Being somewhat od a lucky person I have several guitars and stringed instruments - I play with several groups and several different styles from traditional folk music through C&W/Bluegrass to Rockabilly. We travel all over the UK but never seem to get abroad - so the cases I have covered already tend to be either local to the UK or UK dealers. I have two cases that are ready for covering a Martin (Dreadnaught) leatherette the other a rectangular mandolin case (very battered) but sound.
Anything you can spare would be welcome....
take care all and thanks for the offers of help so far.