GHR & reverse hyper

I can’t answer your origional question, but here are a few other ways to do those exercises.

  1. For glute-ham raise: load a barbell with heavy weight and secure it in place so it doesn’t roll around, you’ll find that the bar loaded with olympic plates is a good height off the ground so you can fix your legs under it, place some old cushions or padding under your knees and under the bar where you place your legs so that it doesn’t cut into the back of them.
    This is how I do glute-hams and it’s MUCH tougher than using a machine. Although you’ll need your own barbell at home, as I doubt you’ll be allowed do this at a gym.

  2. For reverse hypers I simply use an old table raised on a pallet, use a dumbell between your feet for resistance.

These may not be ideal for you, but hopefully you can try them before commiting to building your own.