Sealing a gap between my spa and the flagstone surround.

by Elise
(Houston)

I had a stand-alone spa added (and piped into my pool) a couple of years ago. Instead of the wooden cabinet, I chose to add a flagstone surround and connecting seating wall. There is about a 1 1/2" gap between the top of the spa and the flagstone surround. It absolutely causes no operational issues but it doesn't look very good. At first I used a rubber tube I got at Home Depot and just ran that around the gap. Worked great until the hot Texas sun ate it up. Do you have any suggestions of how I could seal the gap?