cowspace can no longer be on same device as archiso files
cow_label to the same device no longer works (presumably since d0d7eb25). It says, that it cannot mount the cowspace, since it is already mounted (full output can be seen here: https://ibb.co/vZkP7Sg).
mkinitcpio-archiso-59-1 resolved the problem for me.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information