Went to go measure a spare arm from my Caster F8T, but the inner hingepin is a solid tube the whole way through, and you need it to be open? Mugen MBX5T/6T is the same way. Well, here are the measurements for the F8T anyway.
Inner hingepin: 54mm at the outsides, 32mm at the insides (if you cut and removed the solid tube.)
Outer hingepin: 44mm at the outsides, 27mm at the insides.
Length: 143mm
Hingepin diameter: 4mm, inner and outer.
EDIT: I want to say someone had a thread looking for the exact same thing before for a Savage, might want to see if you can track it down.
EDIT AGAIN: Here it is, not as much info as I thought, but it's a start:
http://www.rc-monster.com/forum/showthread.php?t=28110