Here's something I've been doing lately. I found that hardware-store contact paper works great, both for front-end paint protection (although I've largely given up on that, after three years of track use on my current paint), but also for making a quick 'meatball' to put on the doors before sticking on numbers.
Grab a serving plate from the kitchen that's big enough to surround your numbers, trace it on the back of the contact paper and cut out a circle. The adhesive on the contact paper comes right off with the contact paper itself (although a quick coat of spray wax before application would be okay insurance). This way, no matter what adhesive is on the numbers you're using, it's not going to get on your paint.
I've been using matte black contact paper on my gloss black paint.
Usually, I get the numbers on straighter. But I was excited last weekend, since it was my first wheel-to-wheel race.