GVP, FYI I went ahead and got a tube of GE 100% clear silicone from Home Depo, because
the installers swore by it, where I used to work.
I put a few letters together and let them set up for around 45 minutes & it worked great.
full setup time is 24 hours, but even after 45 minutes, there was no way to pull them apart.
I'm not sure if it's necessary or even advisable, but I did scuff both surfaces before putting them together.
Good Luck & please let me know what you went with.