A/C closet not enclosed

Hey guys, I have a house (my house) built in 1990. The closet for the A/C has a direct opening into the attic. I don't have a picture of it but what would be the best way to enclose this closet?

I guess butt something up against the duct on all 4 sides and secure it to the 2 x 4's? What does it need to be, regular plywood or is there some other type of product that would do better?

Thanks for looking, when I get batteries for the camera I'll post up some pictures.

If it's both a furnace and a A/C unit then you would have to be careful as to the materials used.

If it's just A/C then you have more choices.

Personally: I wrapped tin foil around fiberglass and pushed it into the 2 inch gap that i had between my Furnace duct and the 2x4's surrounding it.