>>108674199
Fixed it. I just setup /mnt/tmp. Seems to propagate into the Flatpaks that have access to mount points at least.
# cat /etc/tmpfiles.d/mnt-tmp.conf
# Type Path Mode UID GID Age Argument
d /mnt/tmp 1777 root root - -
# cat /etc/systemd/system/mnt-tmp.mount
[Unit]
Description=23GB Tmpfs Scratch space
[Mount]
What=tmpfs
Where=/mnt/tmp
Type=tmpfs
Options=mode=1777,size=23G
[Install]
WantedBy=multi-user.target