The basic design - two boards that are rigidly fixed in a precise vertical alignment each supporting the pins - is good. But I don't value the Molex and JST connectors. I'd rather be able to grasp more easily onto the top board without having so much stuff in the way.
Fortunately, designing one isn't hard. Just put two 6 pin AVR ISP footprints on a board and connect each related trace. You use a pair of boards to make one fixture, and since they're both the same, they'll both align perfectly. My big hope is that if I use a pair of 1/2" #4 threaded spacer between the two boards and a 1/4" #4 bolt on both sides of each to hold the boards on, then there will be enough room on the lower board to
You assemble the adapter and align the pogo pins the same way SparkFun says to do it on theirs. Start by attaching a 2x6 right angle DIP header to what will become the bottom board, making sure to install it on the TOP of the board, and that it doesn't stick up more than 1/4 inch (so that it won't touch the top board once it's in place).
It turns out that SparkFun's drill holes are the same diameter as the Pogo pin itself. They may be counting on their PCB vendor consistently erring on larger holes. I can only hope that OSHPark does the same. A snug fit is ok, but not fitting at all would be bad. We'll see.
I envision bigger things from this design, potentially. Because the top is (nominally) flat, you could remove the top two screws and screw longer ones in, attaching the whole adapter to a larger programming jig, possibly for mass programming panels of devices all at once. The only hard part of that is figuring out how to get avrdude to individually address multiple USBTiny programmers simultaneously attached.