I have SO many envelopes in my stationery stash, and the real take-home lesson here is that the next time the girls make handmade cards, I will cut down the greeting card blanks that I make for them to the size of envelopes that I already own.
Fortunately, we have many half-used and abandoned handwriting sheets lying around (the children do enjoy copywork, correctly presented--more on that another time), and it is exactly that, or a similar large-format sheet of scrap paper, that will make you the perfect custom envelope for your own quirky card in about one minute.
First, lay the card centered on the paper, but closer to the bottom edge than I have pictured here:
Fold the bottom up:
Fold the top down at first one corner--