here is a quick idea of mine .
based around a minow trap using the bottle neck design of a crap trap and adapting it for a ray or other flat fish .
its designed to hold in the current so the weight could be a simple anchor not necessarly heavy but the anchor will make sure the trap is facing the correct way . the shape and solid front panel help to keep the trap down on the bottom and in the path of the current and the fin helps to keep it stable .
water movment in the trap itself will be reduce due to the shape and so easier on a traped ray but there will still be benifical movement
the rear panel is more solid but not necessirily totally solid to help funnel the scent of the bait the right way
if the trap is longer than it is thin then you could probably do away with a directional fin . if its squar then the fin would be needed i think
the rays would enter through the back and go up as they would the edge of a glas tank . on reachint the top they would go over and into the body of the trap .
getting out they would go up and feel the edge and be reluctant to travel down in a similar way although not impossible . ( much like its not imposible for a lobster to get out of its trap yet the pots still work )
there are no moving parts ( other than the anchor chane / twin / whatever ) and so less to go wrong .
the entrance gaps would need to be tested and worked around to find the optimal size but esentialy i think it would work well and could easily be made into a colapsable desitng or just simply as kit form held togethter with tie wraps or something .
