I have the same Char Griller setup and mine leaks like a sieve too. Best I've found is a $24 u-channel Nomex gasket kit and some RTV600 sealer on Amazon. The self-stick isn't great, so the RTV sealer is a better option. However, it's going to need replacing about every 1-2 years.
I opted to just replace my smoker with an OKJ. Given that I only paid $25 for my Char Griller (facebook marketplace find) it wasn't a loss.