When I made mine I just cut 2 pieces of Reflectix big enough I could fit my largest pot inside ( 2 qt)
One piece I made about 4" longer (flap) so I could fold it over. I used some self-stick Velcro for the flap.

I didn't use any tape to put pieces together. Instead I bought a small tube of Silicone GLUE/sealer and applied a SOLID U shaped bead around the longer piece and then mated them together and put a cutting board on top with a weight and let dry overnight.

I just roll it up after using an secure with a cord.
You could fold it too if that's easier to pack.

Another little tip is when your bringing the water to a boil, and your adding ingredients to a pot, some recipies/packaging tell you to add your ingredients AFTER you get the water boiling.

I don't do that. I add ingredients to cold water and stir it occasionaly as it heats to get everyhting dissloved AS the water gets to boil. Then stir vigorsuoys and put in cozy.

I find at least for the meals I make you get less clumping and a more uniform 'doneness" if that makes any sense to you. ;)