Hey Nick, hey Greg,
you don’t need to find a USB 2.0 flashdrive. It is true that (Raspberry Pi 3 based) Onefinity Controller does not support USB 3.0. But every USB 3.0 flashdrive is downwards compatible and will be used in USB 2.0 mode and will work on the Onefinity Controller.
What might not be supported, is exFAT Filesystem found on practically every USB drive nowadays. You have to reformat your flashdrive using NTFS or FAT32 instead.