... which pda (make/model clues please) as they're soooooo many as well
The mount could be a RAM-B-149Z or 149ZA (short arm) and pda cradle, try looking at RAM-HOl-PD3 for starters.
To put the pda inside a box, then on the market is RAM AquaBox or OtterBox or Pelican cases. AquaBox (eg RAM-HOL-AQ2) needs RAM-B-202 on the back of it so mount is now RAM-B-149Z-Q1 or with short arm, RAM-B-149ZA-Q1. Also checked width of device and width of AquaBox. Otter box also use RAM-B-202 on the back, but do check as some (depending pda) either have hand grip or a outer docking sleeve.