Now I'm not 100% certain about this, so please don't blame me if it just gets more messed up, but would it be possible to "pick up" those remaining pieces with another piece of plasticene? That works at least with blu-tac and white-tac. Though blu-tac doesn't have much of a habit of sticking to surface, it mainly just leaves bits left behind every now and then...
But for the initial question, I usually cover the edges of wheel wells with masking tape, then fill in the centers with more masking tape, blu-tac, or a combination of both. I haven't tried that wet tissue trick, but I'm starting to think it could work quite well combined with the edges done with tape. Best of worlds: neat edges, easy filling of the centers!