Correct... you have to cut them down yourself. For polymer, I would recommend cutting the mag short and rejoining in the center of the shortened mag. A good adhesive with thin wrap over the joint should be ok. You're counting on the wrap to hold the tensile strength of the mag-spring. Flexible urethane or silane based adhesives are generally pretty stout in shear. The mags will hold about 10-12 rounds depending on a few things. I trimmed my mag follower & spring shorter too.
My steel sten mag was cut at the bottom and I rebent the taps to hold the original floorplate.
You can notch the collapsible stock arms so you have multile lengths/positions.
In the shortest position, the overall length gun is smaller. See the comparison below.
Below is a picture of a chopped down sten mag (for mac-11/9) and the adjustable stock.