GunMag Warehouse shows a simple method to add capacity in your Lancer MPX magazines.
The MPX magazine has a floor plate with a protrusion. This protrusion hits the bottom of the follower.
All you need to do is chop the protrusion down and this will allow the follower to move further down the magazine. Do it right you can get five more rounds in your MPX magazine increasing the capacity to 35 rounds.
Now any modification is your responsibility. If you are not comfortable chopping parts of your $50 magazine, you can get the Springer Precision magazine extension. They retail for $35 or $30 each if you order more than five. The website says “DO NOT USE THE BOTTOM PLATE ON YOUR MAGAZINE SPRING” so this seems more like a +5 magazine extension since there is no longer a protruding plate to prevent the follower from stopping early.