I use to mounted eye bolts to my bed frame which was a mental bed frame. Unfortunately the two I did this to I kept bending the bed frame legs.

I forgot to add this but you could get two or three adjustable tie downs long enough to go between mattress and box spring. Cut the Hooks off with a hack saw and get spring loaded D rings that hand a lot of stress. That way you can adjust them to stick out of the mattress and use the D rings with the restraints. Then once play is done you can remove them from under the mattress put them out of the questionable eyes.