In case you're wondering, I posted those plans here). I had looked at prints in stores and didn't find anything that suited me, but I did figure out my style. So I went home to try to find some interesting pictures in our own collection I could turn black and white.
I picked up three shadowboxes at target for $10 a piece. I had the images
scaled to be 7"x7" inches wide printed on an 8" x 10" sheet. It was about $10 to get them printed down the road at Walgreens.
I decided not to waste time hunting around and spending more money to get a real matboard. I just used some thick white scrapbook paper I had on hand, a square, exacto knife (blades) and tape. Pretty much self explanatory, but there's a nice image of what you need. Both the lines and corners came out very nice and straight using the exacto knife.
All finished. Here's a closeup of the images I picked. Sorry that the photos are a really poor quality.