You might consider adding some blocking

Metal plate

How about a thin metal plate that could be tapped.  It wouldn't need to be too thick, just thick enough for a couple threads.

Or...….

Solder a screw to a metal plate.  Glue the plate to the end platform and put the coupler box on the stud and secure with a washer and a nut.

Or.....

Solder a tube that will fit in the hole of the couple pocket to the metal plate.  Tap the tube, put the pocket on and secure it with a washer and a screw into the tube.

ways to mount couplers

on thin platforms? I've mounted them on caboose models by various methods. If you have room to glue on a doubler under the platform ( without lowering the coupler seating surface too much) you could use a longer screw. I've drilled and tapped the platform then cut the screw flush and painted it to match the floor and it's not too visible. I've also used flat head screws inserted from the top thru counter sunk holes in the platform with nuts on the bottom of the coupler box . If you only have room for a thin doubler you could use metal and a shorter screw( you might want to extend the metal back under the car a ways and screw it to the car floor to make it more secure. You might just epoxy a metal draft gear to the bottom of the platform and find it strong enough, might depend on how many cars this coupler must pull? Make your best guess as to the best method for this car then proceed, you can always try again if it doesn't work out the first time...DaveB

Thanks

Sat on this over night and read through all the suggestions this morning and sat on it some more. I am going to try the glue 1st. If that doesn't work in the long run then I will find some sort of metal plate or order some new parts to replace the platform-coupler pockets with and go from there.

As for how much will these cars be pulling, it maybe just the 2 cars, may end up with 3, or maybe none at all. The HOn3 experiment is still on going.

Jack

Before doing the couplers....

Perhaps you've done this, but I'd suggest getting the car mounted, at least temporarily, on its trucks to see what the coupler height you are currently dealing with.

That may open up more possibilities (or introduce more requirements) to getting your couplers mounted at the correct height.

" I'd suggest getting the car

  Good point, I should have mentioned this too. ...DaveB 

I have

This occurred to me Jeff. And since I occasional do stuff out of order I even wrote a note and put it in front of the car. I don't have a coupler height gage. I do have an old Precision Scale shay, non working that in it's working days pulled a string of cars without losing them so I am using that temporarily. So far it looks to be a pretty good match on the height.

Jack

You might want to make your own gauge

I suppose you could just use the Shay, but I imagine it might be a bit unwieldy. But use the Shay to set the height for a height gauge(s) you can use. It would probably only take a couple small chunks of wood and couplers.

" I don't have a coupler

  If all your cars work with that engine then no problem. I find it handy to make a gauge to test the height of the car floor or coupler mounting surface. I just cut a block of wood to the correct height and set it on the rails then roll the car up to it and see if the coupler mounting area is too high or too low or hopefully just right. The correct dimension can be determined from the kadee instruction sheet. Factory made coupler height gauges include a small protrusion to check the floor height. ...DaveB 

Height Gage

Making one is an excellent idea.

Jack

Late to the party

When I built HOn3 cars (in college!) it was published somewhere to consider a brass bar down between the center beams for weight and to provide a place to attach the coupler with a screw. Now I would drill and tap but back then I was “financially challenged” so drilled just big enough for a wood screw to make its own threads in the brass. If you still have room for a 1/4 wide strip then it might be a simple solution to epoxy it to the car floor.