With the thickness of my mounting platform, the short machine screws that come with the TinyDuino Mounting Kit just are not long enough for my stack of 4 boards (5 with the ftdi board) and while there are probably a million ways to solve this problem I though I would post mine in case it helps someone else with the same issue.

I found some 3/64 inch brass rod at my local hobby shop [made by K&S engineering, Chicago stock 3/64 brass rod stock number 1619] , which cuts easy and threads perfectly through the plastic standoffs in the mounting kit. So I simply shape this into a "U" shape, and run it under the mounting board so that it pokes up through two diagonally opposite holes of the tinyduino stack.

On the other two holes of the stack I use the supplied machine screws with bolts, and this afixes the stack to the mounting platform, but these are then two short to hold the upper boards. But the brass rod is as long as I need to make it, so it supports the upper boards. Then to lock the upper boards down, I thread some very small diameter heat shrink tubing down the brass rods, and once shrunk in place this holds the upper boards in place.