The grill has a small inset and four screw holes, one in each corner. I never thought of using hinges, that's a good idea. I'll have to check the grill and see if it would work.
I do plan on sealing and insulating the return for winter, maybe using that magnetic sheet to cover the hole. I like those inserts you have linked above, looks like they are build for hard wood, would they work good in soft wood too? Thanks again