That's a good idea- the gaskets for the 128H 12" woofer should be the same thing, IIRC. That was used in several JBL studio monitors (the 4312C included, IIRC)... so they should be attainable from the JBL pro side...
Unfortunately, I don't have any left around here now, or I'd offer 'em up... however, one thing I've done, in a pinch, is get the gaskets for a Cerwin-Vega 12" woofer, and "peel down" the laminated cardboard gaskets to where they're only about 1/8" thick. They should fit on the 122A OK then, and they only have 4 screw hole lands, like the stock JBL ones...
Regards,
Gordon.