The shock cord and anchor are fire resistant. I used this set up on my BT-70 Dragon Rocket without a single failure. A nice feature of building this anchor is it can be flexed to fit any mid power rocket tube and it's really cheap to make. The anchor is installed 3 inches into the tube and glued into place using thick super glue and epoxy.
This is how you get the curvature for the anchor to fit your tube. Once in place pull about six inches of the shock cord thru towards you. Now use thick super glue lightly and put some in the hole in the rubber and along the edges of the cord that touch the rubber careful not to glue the first part of the anchor to the tube. Then spray it quickly with some accelerator remove quickly again so you don't glue the rubber to the tube.
Once you have the first part of the anchor built it is just a matter of sliping 3 more rubber key covers one by one over the long end of the shock cord bend each cover to the curvature of the first part of the anchor and super glue each of the key covers together. Once you're finished you should have four key covers glued together and the anchor should have the correct curvature for your tube.
Now take the six inches or so of the shock cord you pulled thru in the first step and tuck it into the other end of the anchor tightly make sure you don't cover the curved side of the anchor . Super glue the end you tucked in so it looks like the picture. Cover the cord that goes over the anchor with epoxy. Next paint this side of the anchor with VHT Header Paint it can withstand flashes of 3000 degrees. Don't get the paint on the bottom or side of the anchor .
I install the anchor in the tube by first dropping the anchor into the tube 3 inches and marking the cord with red sharpie. Then on the curved side of the anchor I put epoxy into the key cover holes and thick super glue around the edges drop it into place and hold it down with two popsicle sticks that are glued together. Now you can put epoxy along the edges of the anchor using the popsicle sticks. In the picture you can see how well the anchor fits the tube It's not coming apart.