Note to Vern:
Did it Vern! Disassembled the MP assembly on my Mellow Tone, which was kind of leaky around the bottom, measured the thickness of kitchen aluminum foil with a digital calipers I have lying around the place (0.001"), cut 2 strips to fit inside the slide carrier, pressed the slide on top of them and took a look.
The top of the U-channel sides were both too high, one side worse than the other, and different in height end-to-end - strictly a machine-made harp with no handwork, I guess.
I draw-filed both flanges to the level of the slide. For those who don't know about draw-filing, it's a method of filing metal that produces very accurate results; you lay the file crossways on the work - picture a cross, the vertical member being the piece to be filed and the cross member being the file lying on top of it, then move the file back and forth along the long axis of the vertical member of the "cross". This allows very slow and predictable reduction of material. I worked as an aircraft die maker for a while in my mis-spent youth. We made rubber-press form dies for fighter plane and helicopter parts.
When I filed it down to the level of the slide (took some courage), I polished my work with crocus cloth. No Butcher's wax on hand and rather anxious to see what I'd ruined, so I washed everything under soapy water and re-assembled the Toots.
Worked perfectly! Now the harmonica is nice and tight! Maybe I'll go into business; "Air-tight Tommy!"
Anyway, thanks for your encouragement, Vern.
BTW, I didn't fasten the work down as you show in your photos. Draw filing exerts so little force on the piece that I just steadied the work with my left hand and filed with my right. Did the full length, minus the locating bosses of course.