I did, but if I was going to start from scratch I'd potentially do it differently. I installed a piece of right angle metal above the fire box parallel with the lid and sealed it all the way along with flat, On the other side with the tray I couldn't do that and stuck a piece of flat across both of the corners so that when it closed they brushed past each other and seal instead. I wish I'd done that on both sides, but you could also use the thicker rope to get a good seal. On the back I stuck flat to both the bottom and the rear of the base so they slid next to each other when the lid closed. Obviously stuck seal around the entire firebox lid because so much heat gets lost there, sealed the whole thing with high temp silicon as I built it, and sealed the firebox vent door up although this probably doesn't matter as much. If you look at my original post you can see me maintaining 240 Fahrenheit no worries, and I did this all day doing a brisket once. The maximum temp I've had in inclement weather was about 150 C and that was with an entire firebox of lit charcoal, both vents open, sat in the rain, smoking some sirloin for a couple of hours
The biggest issue you probably had was dumping the charcoal on the base of the firebox. This offset style is really sensitive to having the lid opened as well, how many times did you open the lid? And what was the ambient temp?
Look at the first few posts of this thread, too see pictures of what I did, if they're not on this thread, they are somewhere on her, clamps for firebox and all sorts.