I've seen a "kit" at lowes for just this.
I've also seen a pink styrofoam lid for them that was super cool. I don't see them online . . . The ones at my local lowes are 1 piece.
Here's one
Energy Guardian Access Hatch Insulation System - For Standard Attic Hatches - This item ships for free. - Energy Guardian Insulated Access Hatches But you could just build one like that . . .