To figure out the design, I measured the base of the fern and added some room to grow which gave me the required surface area. Then I measured the width of the cedar boards we bought and gave 1/2" between the boards. At this point, you might be getting a bit confused, so check out the illustration.
I needed a surface area that was approximately 12" by 17" originally, but when I laid out the design, it was much simpler to make it a little larger so that the numbers were nice and even rather than odd portions of an inch. By increasing the width to 17.5", I could use 9 boards cut to 12" long and spaced 0.5" apart. The runner boards then became 17.5" and run along the back.
Each board got 4 nails into the runner boards (2 at the top and 2 at the bottom), and hangers were screwed into the back and attached to a short length of chain with a couple carabiners. The chain is a bit overkill right now, but these plants get big and heavy.
Then to "attach" the staghorn fern, I stuffed some moistened moss into the gaps between the boards, mounded up some more moss and orchid bark, and laid the fern onto of the heap and continued to stuff around it until it looked smooth and even. Then my SO held it in place as I maneuvered fishing line around the bulk and stapled the line to the board. Once secure, we held it up, and I stuffed some more moss where a few gaps had appeared. Eventually the sterile fronds (the pancake ones at the base) will cover the line, and no one will know it's there except those acquainted with mounted staghorn ferns. Through this process I actually uncovered the original fishing line which was kinda cool.
So if you're ever thinking about buying a mounted staghorn fern (~$45), save yourself some money by buying a potted one (~$8) and mount it to a board that's sure to last a VERY long time!
For those not acquainted with staghorn ferns, they are very easy to care for and put on lots of growth during the warm months. They need minimal fertilizing and can take a good amount of drought - mostly they need humidity. They are also epiphytic and are accustomed to growing on the sides of trees. Some people around here have monster baskets of staghorn weighing 400-600 lbs and are too large to remove from trees, so they get covered hanging in the tree for the winter. My ferns go in the garage and get watered once a month or so. This mounted one will probably hang on my fireplace like a trophy.