I have designed and built a Apple ][ card that can interface two SD cards as ProDOS block devices. It uses very commonly available chips, and is based on the ATMEGA328P microcontroller and 82C55 peripheral interface. It uses simple readily available parts to be easy to reproduce, now and into the future. It can host ProDOS order block devices from either raw block device SD cards or from FAT filesystem formatted SD cards. I also have planned interfacing the card to a Wiznet 5500 for Ethernet support.
You can find the github site with the gerbers and firmware at:
You can also find many other projects I've done on my github as well.