Making a bedrail

What about using electrical EMT conduit and clamps?
You can drill it easily and attach it to what ever or you can smash the ends in a vice for a flat surface to mount rails to.

Small blocks of wood for spacers installed along the front spaced apart the width of the upright rail pieces on left and right and then a solid piece of wood left to right installed on top of the spacers. Do this at high point of front and another set at low point .You will now have a place for the uprights to drop down behind. 3/4 in finished plywood would be a good choice, sand the edges slightly to ease the corners. Very strong. Hope I explained it well enough.

I'd considered EMT but was hoping for a more finished look and it needs to be removable. I'd checked out fittings for boat railings but have not been able to find one that doesn't have a hole exiting at an angle. The boat tubing is 7/8" inch and I could get 90 degree fittings but the part that mounts to the front edge of the bunk is the problem. (see my other post below about Class A curtains to see the bunk in the down position).

A trip to a big box hardware store should provide you with some ideas. A couple of short pieces of 1 inch iron pipe maybe. You could get a piece 8 inches long (there abouts) drill a couple of holes near one end, one a inch above the other. And then use long screws to go thru the holes into the wood end. That would give you some rough and ready sockets. Some long 1/4 inch machine bolts that went all the way thru the wood end, and with some big washers, would be a bit stronger.

If you want pretty, cut off the screw threads, file or grind the ends so they are smooth and beveled, and apply some nice paint.

“but was hoping for a more finished look”

I’m the most unfinished look in my TT.

There are lots of these sorts of things available if you look around a little. For instance, these may work:

Mortise Bedlocks
Mortise-free Bedlocks (these don't look quite as handy to me)
Taper Connectors (these look quite promising)
Titan Railing Connectors
Striplox Surface-Mount Connectors (these also look promising)
Panel Connectors
Flush-mount Clips

A lot of nicer toy wagons have removable side panels that are held on with a simple U-shaped bracket that a square post slides into. Something like that could work out well, too. They're (currently) unavailable here but shouldn't be too hard to manufacture if one has access to a decent vice or sheet metal brake (and can't find something similar suitable).

Another possibility besides a solid rail might be a cargo net that could be anchored to either wall and the front frame of the drop-down bunk. Do take care to make sure the net openings are appropriately sized if you have little children sleeping there to ensure that they can't get their head caught in the netting.

Have you considered a foam bed bumper? Amazon has a variety of them. I believe they require either a tight fitting sheet or mattress pad.

How about something like this?
Stake pocket
There is a smaller version for a kid's wagon, but I haven't found it.
Further search turned uo these.
wagon bracket

you could modify one of the fold down bed rails by cutting off the part that goes under the mattress and mounting it with spacers to the bed edge. one like this? Safety Bed Rail

Look at Speed-Rail, different fittings and mounts (wall. floor) are available. You can also get painted Speed-Rail tubing for more finished look. You can replace the fitting set screws with thumb screw or lock pin for easy removal. That is what I used on my storable Bunk Bed project.