I have the same problem. '72 boler
and 165lb "kid" to sleep on it. I don't trust the little fiberglass shelf just under the window to support the bed so I have been thinking of the following solution:
1. For vertical load, I will build a frame out of a combination of 2x2 (for the vertical posts) and 2x3 / 2x4 (for the horizontal bits) cedar (lightweight, doesn't warp, smells nice) and let the vertical posts line up (more or less) with the metal frame underneath. This will prevent the posts punching through the plywood, not that it would, but lets overengineer it.
2. For horizontal support (don't want the whole thing collapsing sideways when my daughter turns over in bed), I will likely just run some 45 degree angles between the posts, like they do for decks.
2a. See picture below.
Instead of the 45 degree angle supports, I could have my vertical posts a little longer, attach the mattress supporting board about 8 inches from the top, and then use 1x6 fencing board (again, cedar) attached so that the tops line up. So, starting at the mattress support board and adding a 4 inch mattress on that, the bottom of the fence board would overlap the mattress by a couple of inches, and then there would be 3.5 inches of the board that is above the level of the mattress. this is to prevent a sleeper falling out. If you are handy, then cut the board so that the area by the head and torso is raised but the area by the legs is level with the mattress - to aid getting in and out of the upper bunk.
The walls of the boler
will provide left-to-right support, the front wall of the boler will provide backward stability, so all you have to do it provide enough stability so that the frame doesn't move forwards, as viewed when standing in front of the bunk. Screwing the fence boards, esp the ones by the sleeper's head and feet, properly onto the vertical supports will provide this stability.
Won't be the prettiest job but I don't want to make a permanent installation that is pretty, expensive and time-consuming because in a few years' time, it will just be my wife and I (i.e. we've gone from sleeping 4 to sleeping 2), and the front will likely receive a remodel then.