I also use the mouthpiece from a 901 on my V4L cartos. I just take the batting out of the mouthpiece first and use a heated needle to widen the air hole on top of it. Then I remove the cap and rubber ring from the carto, pop on the mouthpiece and vape away... makes refilling the carto a breeze and is more comfortable to hold in my mouth.
I did try the pen/condom thing out of curiosity. I used a retractable metal pen so that I did not have to take it apart, just retracted the tip. The pen perfectly created a hole, but my aim was off so I had to overlap the 1st hole and take out a half moon to center it more... just for aesthetics, LOL. The I rotated the pen tip back up and the small condom pieces popped right out... seems easier than dissembling a pen. Vaping with it is fine, although a bit more of a mouthful, and it gives your teeth something to grip on to as well as protects your lips if the carto should become too warm. Although I'll use it from time to time just for a change of pace, I think I still like my 901 mouthpiece better.