Dennis - your inbox is full, but here's my message anyway
Thanks man. Sorry to be a pain. I think foam tape is really what's called for here rather than the transfer adhesive I've been using, since the bumper is ever so slightly contoured and it really needs that buffer to make it stick well. But these letters are so much thicker than the ones you said you have, there's just no tolerance for the grey trim tape I'd normally use for such a thing.
Will give the glue a try this weekend I guess.