I cobbled these together in 10 minutes. They seem quite sturdy, but I wouldn't stand on them.
two boards ( I used 1"x 8" x 46")
two uprights ( mine ended up 27" tall; see "upright height")
upright Height = (Distance from floor to top of box springs) + 6". If you will only be using this on one bed you can cut the uprights to length and be done. Otherwise you must con$ider click height adjustable uprights. Vintage legs could be sweet. With flip click lock brackets for storage.
Assembly and Use:
Attach upright about 3" from one end of plank. I used a 1 5/8" drywall screw. Wedge about 18" of the other end between the box spring and mattress. Space apart as comfort dictates.
You may want to sand these before putting to use. You could pad them and cover with vinyl. Upholstery tacks around the edge. Or not. A splinter might need "invasive surgery" from a "doctor". Immediately following the massage.
Disclaimer - Not responsible for accidents, injuries, or babies.