Well, my quick reconstruction is done. I like the results, in theory. In practice, apparently double and triple-checking my measurements wasn't good enough.
I turned this...
...into a new checkbook cover, because the cheap plasticy one that it came with that I've been using since I got the checking account at the beginning of college was completely shredded. That, and I didn't want to throw it out, even though parts of it were ripping at the seams and the zippers were basically shot from my habit of hauling a book everywhere I go. (Bad pattern choice. But I love the fabric.)
....and the checks fit in there fine...
...but I decided I wanted to make some card slots too, since my current purse has all those pockets to eliminate the need for a wallet, including enough card slots for the essentials, but not enough for others like my bank/insurance cards, frequent buyers, etc. As you can see, I forgot to factor topstitching into my measurements, and so the cards don't actually fit horizontally like they were intended to.
It also means I probably should have made the cover pieces wider, because the middle one sticks out funny. That, and I should have used more elastic for the loop, because I can barely get it over the button. Sure, I could fix that, but I don't really feel like undoing the entire inside of it to add a bigger piece of elastic. I'll probably just end up moving the button itself eventually, but I'm too lazy to do it right now.
The moral of the story, which my several attempts at patternmaking should have taught me already: Never, EVER trust my math skills.