[Slalom4me]

Thank you, chrisguyw.

A few comments in reply.

1. I think my Thunderslot wheels are the same as the so-called 17" NSR 5004's.
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   Paul Gage offers urethane tires for Air System wheels that are molded such that the gap between flanges is filled.  His nomenclature denotes this feature by appending a "DF" suffix to part numbers.  ie:  PGT-20125LMDF (Regular compound; 20mm OD, 12mm width, 5mm hub rib width, 17mm ID, dual flange hub rib.)  An incomplete list of dual flange sizes can be seen on the NSR pg of the Paul Gage eBay store.
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2. For those interested in pursuing the approach of filling the hub gap with foam, here are some alternatives to the labor, expense and waste associated with cutting up perfectly good sponge tires.
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   • Use ready-made rectangular or round profile closed cell foam strip/tape sold for use as gasket/weatherstrip applications.  (The round profile is typically referred to as "Gap & Joint Filler Backer Rod.")  Available in various widths & thicknesses & dia. and densities, some suitable for filling a hub gap - eg:  Frost King Vinyl Foam Tape 3/8"w x 3/16"T Moderate Compression.  I do not know whether the lack of continuity presented by a strip, compared to a donut is a significant consideration - perhaps ...
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   • Use hole cutters to punch flat washers from a sheet of closed cell foam of suitable thickness and density.  Fashion pieces of suitable diameter thin wall K&S Tubing into hole cutters for the ID/OD of the washers intended for filling hub gaps.
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   • Mask off wheels, leaving just the hub gap exposed.  Then apply expanding Polyurethane Foam Sealant from an aerosol can as sold for stopping drafts around window and door frames - eg:  Red Devil 0912 Foam & Fill.  Let set, then trim.