Very few ray keepers appear to be able to house them at that size and larger because it requires a tank bigger than the usual largest tank of 8ftx4ft. It looks like this is the answer to your dismay at the lack of success in rehoming.
Anyhow, we are only getting in the ray game because we just got our first ray permit from the state of FL and IDK much about them but learning. She could have been first ever at our Aquarium Attraction. I'll stop here. If I continued explaining myself, it'd be inconsiderate to your goal.
GLWS.