I've got 2 18" WSM's. For the first one, I bought the Cajun Bandit door and it works great! I've had it for years. Thanks to this thread and WSM forums, I also use gaskets, the Arbor Fab basket, a BBQ Guru w/fan and adaptor, Fireboard w/the BBQ Guru fan, and the lid hinge. The Fireboard is awesome! Gaskets are a must. The lid hinge is a bit wonkey, but if you don't have a lot of space to set the lid on the ground it is a nice accessory.
I bought another WSM to keep at my Uncle's farm for get togethers, and I treated the door, bottom, and dome with some extra gasket material I had. Works like a charm. I've burned it in and done two long cooks on it and it does not leak.
I highly recommend this kit for the base, door, and dome. It's enough for 2 WSM's and no leaks:
https://www.amazon.com/Gasket-Weber-Smokey-Mountain-gasket/dp/B00CI6WFCU/ref=asc_df_B00CI6WFCU/?tag=hyprod-20&linkCode=df0&hvadid=312142484282&hvpos=&hvnetw=g&hvrand=12971683097946288872&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9028274&hvtargid=pla-592593434834&psc=1
Fireboard (skip their fan. it's a PITA):
https://www.fireboard.com/shop/fireboard-2-drive/
BBQ Guru Fan:
https://bbqguru.com/storenav/Index?categoryId=&&productId=403
Adaptor:
https://bbqguru.com/storenav/Index?categoryId=&&productId=6
I got the BGE adaptor as well, but I haven't tried it with the Fireboard yet. I use the BGE for high temp cooks, so it hasn't been a priority to smoke anything on it.
I'm still fiddling with the Fireboard, but once I get it dialed in it will be perfect. The app is great and the damn thing is precise. WiFi and Bluetooth. I also got readings on LTE when I ran to the store while the food was on. I know it is pricey, but it may be my favorite gadget I've purchased in the last 5 years.