Ive bought a 'sorry for your loss' greeting card and write 'I quit ' on the inside and hand it to my superior before leaving when they asked for my notice in writing. I already had another job, I didn't need a reference and if I did I'd just have a friend lie for me. Fuck the bosses.
They just need that for their records, so next time you should get one of those giant office sized birthday or sympathy cards which won't fit in any standard drawers. They can still scan it but it's annoying.
If you're feeling especially grumpy, sign a bristol board with 3" tall letters, or novelty cheque sized document (you can spend $2 for the bristol board, or like $50 on novelty stationary).
If it's cardboard or paper, they could just fold it. Use something they can't fold, like acrylic sheet or super brittle paper-like that would crumble or break if folded.