SG Gels is great.
Two part epoxies are also usefull n certain applications for securing frags,that may already be on a plug, small rock, shell etc, to larger rocks. SOmetimes using two part and SG gel is the best option. I never use 2 part directly on SPS frags; seems to heat up or somehow damage the base if directly used on sps but just my experience.
May two parts are available online but I use Aquamend from my local hardware store. 2$ instead of 9$.