MD7
There are no set dimensions for a depinger. All you need is something to creat a blockage. A nice big wad of gum would work

. Seriously, if I were machining one I'd make it about 1.050 in diameter and 1/2 " long in an "I" shape. I would use just 1 oring, 1" OD x 3/32 cross section. This design would take up minimal volume in the airtube. You'll have to figure out the exact diemnsions of the Oring groove and such once you get it apart.
Teryx