For a tekkeon, I asked them a while ago and they said the Panasonic Toughbook plug (it's a longer C tip IIRC) and it fits without modification and I never had mine fall out when I used it (it did have some resistance, it wasn't like it just sat there). A wicked snug fit requires you shave about 2-3mm of black plastic off of the B tip since the barrell is long enough to make contact but not long enough to snap in and stay in on it's own as the plastic prevents it from going deeper. A number of folks don't modify the tip and rubber band it to keep it in but mine wanted to short out periodically when I tried that.
edit:
Only recommendation is on powering, feed it as many volts as possible without exceeding 24v. Your headroom is based on the input voltage (for better or worse).