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.