The CHERI team had the same problem: https://github.com/CTSRD-CHERI/cheribuild. You'll need something along the lines of "./cheribuild.py sdk-riscv64 freebsd-riscv64 disk-image-riscv64 run-riscv64", bit of disk space, and a free couple of hours for it to finish building everything; at the end you'll get a login prompt for the guest.