Finished it!
Made up 4 hooks out of 3mm thick 50mm wide galvanised steel. Made up a block to sit in the hook so the face that mounts on the tub rests on the blocks and the lip of the top drops down beside the block.
And now for the inaugural unedited test... (after it was hanging about 2in above the tub being bounced around for half an hour just to be sure!)
For @davewaldo
And just to probe it works, after all installed I dropped it back to the floor.
Plenty of room underneath for it to not get in the way (if I reach up as high as I can I am still about 150mm short of touching it!)
Not happy with the angle, the rear ropes need to be shorter so will drop it down tomorrow and redo the splice on the rear ropes so they are about 50mm shorter. Will also make up a bar to lock into the front mounting holes to stop it spreading whilst suspended and may use that to store the front panels on rather than on top.
A good days work