I've cut down several 308 pmags to 5 rounders. It takes maybe 20 minutes once you figure out how to do it. Here's how I did it.
First, cut down the body just below the first rib. This will keep it long enough to stay below the mag well. With this length it still holds seven rounds, so you need to make a block. I just used a piece of the waste material from the bottom portion of the mag. I made it 1.7" long, and fit it in the groove in the bottom of the follower, and pin it in place with the mag spring. It should look like this:
Two notes on that, I wrap the block in duct tape to keep it from rattling inside the mag when it's done. Also cut 5 wraps off of the spring, still gives good tension and perfect function.
To re-attach the floor plate, I chose to drill and tap the plastic and use 6-32 screws. This way it's easy to take down for cleaning if need be. It turned out like this:
Note that I didn't take the time to make sure the screws were straight across, a sloppy mistake I have since corrected. Those screws need to be about 1/4" long.
A final pic of the finished mag next to a 20 rounder. I've also made a 15 by cutting one off just below the last rib. 15 is plenty for me and allows it to work better from a bench than the 20's. HTH.