Weber 1201 Tool Holder

With sealing being more critical with a smoker and cooking low/slow than grilling with a kettle, I would not add that accessory to the WSM. As the WSM is used over time, the guk bills up around the lid and center section and you do get a pretty tight seal.

Those mounting tabs don't look to me like they would go under the lid. If anything, the angle looks like they would tend to settle into the rolled edge of the lid instead of the flange of the top of the middle section. Maybe they could be bent to a tighter angle, but as they are, I think they would interfere with removing the lid more than anything else.
I bought one of those for my kettle a couple of years ago. My lid didn't fit right with it on. I would skip it. Mine is now attatched to my work table it works fine there.
This unit does not fit on the WSM. I have one, I tried it and it doesn't. Besides it's easier to put a table next to the WSM and keep your tools there with your therm.