How do I ensure a level, seamless capstone on a retaining wall? [View all]

The wall was made out of rough, Rumblestone, but it's as level as this type of paver allows. The capstone is Travertine. It was an obvious great buy, probably the last of a large order and we just lucked into it because the buyer wanted to move inventory. It takes 5 of the Travertine capstones to cover the top of a 120 inch wall. A couple of the Travertine tiles move a bit, but if we gently life one side of the stone, it all fits seamlessly.

We were going to use Loctite PL 500, but I have a hunch we are going to have to use cement. Maybe Heat Stop, because it will be four feet from a firepit?

https://www.amazon.com/Mutual-Industries-60090700-0-0-Heat-Stop/dp/B00XNQL6GK/ref=as_li_ss_tl?ie=UTF8&qid=1466781800&sr=8-6-spell&keywords=heatstop+mortar&linkCode=sl1&tag=shiyouligblo-20&linkId=5f53c14ef7a1443decc3c68e134377e6

Am I on the right track?