I tried some new bolts in the vertical adjustment and made sure the tabs were fully bent out (per the "Servicing Truck Cabs" instructions) and it is thankfully doing a better job of sealing now. But I don't think the gasket modification is a great one, considering it allows water to sit in the space.
Edit: Located one direct from Steele, shipping was "tolerable". The joys of international shipping, especially these days... but It's on the way!
