I think the chances of finding one that would fit at salvage yard are quite small. I would think your best course of action would be to construct one out of wood and epoxy and install the back on hinges so that it can be lifted at the front as opposed to be able to slide back. you may install some hasps at the front so it can be secured when away from the boat. Some guys have made some nice pieces of work to replace the cabin hatches and I can't see why not a nice companionway hatch can't be made. The other alternative would be to have someone with a 36' remove his hatch and lend it to you so a fiberglass mold can be made. The process is not cheap and perhaps you may find that they be quicker to donate a kidney than the use of a hatch.