You can buy them for around $100 and install it in about 10 minutes. It is really simple. The greater benefit is helping to keep hot attic air out of your house.
Put one in our upstairs rec room for the attic opening. Amazing temp differential before and after. I have had the zipper version and the box version that you just lift out, both did the job well and helped keep the attic space more isolated from the inside.
Newer pull down attic doors are insulated and weatherstripped so depending on how old yours is, I might not make much of a difference. Our house came with the zipper attic tent and when I replaced our 50yo wooden ladder and door with a newer one I just left it in place. Probably overkill but I figure it can't hurt and keeps dust/bugs/insulation from building up on the ladder and falling on my head when I open it.
I did something similar. I've got several closets on the second story with attic access doors. Built some covers from foam panels and the temperatures decreased dramatically in those closets.