We use PiCAN3 hat on our DEATHFISH 2 demo car:
PiCAN3 CAN Bus HAT for Raspberry Pi 4 with SMPS and RTC (copperhilltech.com)
DEATHFISH 2 - General / Project & Dashboard Showcase - RealDash Forum
Its handy as it can power the RPi from 12v input to the hat.
As for other CAN hats, I believe they all use SocketCAN connection and thus should be compatible with RealDash.