A wooden base, longer than it is wide, with a leg at each corner (multiple locations for length adjustability would be essential).
2 lightly-cushioned slings (neoprene, perhaps – think broad camera straps – with a slightly stretchy fabric backing. (Note: Would have to be washable to deal with any leakage that gets on to them.)
This would support the lower leg at the heel/ankle and back of the knee, without putting any pressure at all on the fluid-damaged calf muscles.
This is the only way I, and doubtless many others, could elevate the affected leg(s) without inflicting more excruciating pain on ourselves, or over-stressing the knee joint.
Such a device is not rocket science (it took me 2 minutes to come up with this while arguing with the nurse about the impossibility of elevating my leg), nevertheless, it appears not to exist. Why?
NB: it can also be made from metal tubing, which would be lighter and more suitable for commercial production.
I offer the idea free to anyone who wishes to make it a reality. I, unfortunately, cannot.