We're wanting one for our double spare bed - so that if we need to co-sleep, which I think will be forever on and off, especially when she's ill - she's safe. I'm happy to sleep with her whenever, but I definitely sleep lightly when I'm worried about her falling out. All bed guards seem to be for toddler type beds.
^ was coming on to recommend that one above, it's great, I use it for cosleeping in the spare room, it's easy to shift between rooms. Older daughter used it for a good while when she went into a bed from a cot.
Try putting a swimming noodle under the fitted sheet at the edge - you need a flat sheet as well to ensure the noodle can't fall down either. So it goes: half of flat sheet, then pool noodle, fold other half of flat sheet over pool noodle, then fitted sheet on top. So long as someone is sleeping on the bit of bed with the flat sheet, the noodle stays in place.