Darkmind...... which style 9mm upper are you planning on using? Oly or Colt? Im surprised the others didn't ask that first, I think they assumed you were gonna be using Oly becasue you mentioned Sten mag usage... anywho.... I have a two peice, locking and removable magwell block system I orginally developed during the AWB for inexpensive sten mags to be used in the Colt style 9mm's. By removing the ejector.... it basically becomes a Oly style magwell block. Yes... the Oly style will feed reliably form the more rearward mag position via the feed ramp/bridge on the forward magwell block.
for several reasons posted in other 9mm upper threads here... I personally have a preference to see you use the Oly style... The KKF(#1) and SOCOM(#2) blocks are pretty decent if you want to learn a new manual of arms by using a thumb lever to changer your mags. If you want to use the standard mag release button and don't want a bulky block welded on the back of your magazine like the factory Oly 9mm mags, then my magwell blocks are your next commercial option. It's very similar to the factory colt mag blocks in concept, but removable.
~D~
SNS