For better durability of the external rig, I'd be more tempted to leave the fill valve as-is or plug it, and drill/tap an new QD air fitting out the back of the mag farther up so that the risk of ripping it off the bottom and possibly destroying the mag in the process the minute go you prone is minimized. Having the hose where it is -- sticking waaaay out the bottom -- is fine for the "shop" tests, but practically speaking, would not survive on the field. Better to try and tap the mag for the air hose in a more protected location or use a 90 deg elbow with QD connector. WA mags already have their fill valves on the back so it may be easier to do this kind of mod with their system. Oherwise, the ROF is astounding (need a box mag) and of course a HPA tank lasts a long long long lonnnnnngggg time given the low consumption rate and working pressure of these GBBRs. Nicely done...
'Fly