Easy to do. If you're like me, you can probably use some of the junk you already have lying around your garage.
Sounds like me, except we have a single car garage which my wife parks her car in.
Once in a while, usually in the fall, when bigger coats start to become the norm, she will start to make suggestions that it is getting a bit of a tight squueze to get into the car.
I take the hint, and either landfill the accumulated stuff, or take the worst of the lumber, usually scrounged in the first place, and cut it down for burning in the fireplace over the winter.
Of course there is always the stuff tucked up into the open rafters and in bins on the overehahd shelves.
But that stuff gets me when I go to wrestle the canoe out of its' rafters storage spot in the springtime.
On the insulboard front, I found I needed two thicknesses of 2" thick, but this was only with the pink stuff. Blue rigid foam 1" no problems.
I have actually used foamcore board with the paper on one side painted matt black, facing out, and it worked fine.
It comes out easier if you put a loop of packing tape under the bottom edge to give you something to pull it off with.
Stapled extra lengths of 120 backing rolls around the edges to make the light seal.
Foam core is super compact to store when I actually want some natural light flooding into the laundry room adjacent to the darkroom.
