I know, that's why you do the other things to help it hold heat. You can make a new firebox out of thick walled pipe, that might help but there's a reason these are called COS=cheap offset smokers.
It's a starter smoker and a BBQ grill at best, the rest is up to the user to make it work. That's why people who smoke all the time buy a good, thick metal offset. These days it doesn't pay to spend 1K on a good offset considering all the options available that are basically an easy bake smoker that takes less time to run and burn less fuel.
You could always use it to smoke for the first 3 hours then finish it in the oven and that's what I'm gonna do from now on but I'll use the pellet smoker to finish it.
OR YOU CAN BUILD ONE!! I know you have the tools and you can take your time to build it the way you want it. A custom offset with a vertical box for low heat smoking is what you need to do sausage and the other smoked/cured meats you make.
Check out this guy from Louisiana that makes them to sell in his back yard. If you dig through his channel he explains all the tips and tricks he's discovered along the way.
https://www.youtube.com/watch?v=T-nd32AU5IQ