PCB version of this, several people suggested I look into PetSD. Pet SD is no longer in production, but was a SD storage system based on SD2IEC, the SD card IEC disk drives that are very popular on the Commodore 64. This gives it support for various commands and file types not supported on the petdisk, and .d64 disk images etc. The original Pet SD had proper IEEE-488 bus drivers, an ethernet interface, USB-serial interface, real time clock, LCD display etc., I had ruled that out when looking around initially, and started instead with the simpler solution. It seems several people were interested in bringing that back, and had been working on new versions. PetSD+, a respin of this, without the ethernet and USB, and PetSD duo, an even more ambitious version with a larger LCD display and an IEC interface for Commodore 64 computers. So I'm now looking and producing a cut down version of Pet SD, in a similar form factor that the ones I've previously built.
Update: pet microSD is now available: