Choose your cozy material according to how much insulation you need.
As a slow learner, I burned my mouth too often on hot food, until I realized I didn't need such thick insulation (reflectix, 3/8 foam). For my FBC (freezer bag cooking), I'm mostly cooking small pastas, so I'm just using cut down insulated mailing envelopes. The larger the mailer, the larger the bubble size of the insulation in it.
I've still got reflectrix for my wide pot cozy and pot lid for canister use in the winter. I'm trying to avoid the weight and the escaping glue on duct tape.
Flat mailers for FBC are not as efficient as a cylindrical cozy on a beer can pot. Again, choose your cozy material to match what you need, since thicker is heavier so it is not always better. I hope you learn from my mistakes.