Choosing right size hide

I think my snake might be getting too big for his hide. Curerntly has some cool looking rock hides but he has to squeeze to get through the opening. He still fits inside but just barely.

He is about 32" long and weighs right on 800 grams.

Thinking of getting one of these but not sure on what size, I think some people on here have used these hides before so any opinions on which one would work best in my situation?
http://www.reptilebasics.com/hide-boxes.

Should also mention he lives in a 20gal tank.

I would either get the mediums or maybe even large.

A medium will work just fine for an 800 gram BP.

So I put it off for long enough the hide was starting to sit on top of my snake instead of him sitting under it. Unfortunately the medium looks just a bit on the small side and the large is too big to fit in my tank (20gal) so this is what I came up with. Luckily my wife has a shoe problem and has an endless supply of boxes.

Cut the lid off, cut the box in half, and cut an opening.
http://db.tt/kG6hx0FT

Stick the two halves together and adjust the size.
http://db.tt/nTDmxym6

Not as nice looking as what he used to have.
http://db.tt/aDZKl2d9

As you can see there isn't much room for him. I think snug is understatement.
http://db.tt/cd9Zj5Jg

He headed right on in
http://db.tt/XOkOV3uS

And still pokes his head out to see what's going on
http://db.tt/c8LNgGRg

Very creative!

You have a beautiful BP, by the way.