This should work for any amputation at knee or above. Not sure there's enough pant leg left for lower amputations.
Warning - This project requires sewing with a machine or by hand. Sewing uses sharp pointy things like pins, needles, and scissors. The writer is not responsible for any damage you do to yourself or your customer or your customer's pants.
Step 1: Test Pin for Fit
For my customer, we already had all his sweats pinned at his preferred length, so I just went with that. If your customer hasn't done that, pin the pants and have them wear them around for a couple of days to make sure they are right. Folded pant legs do weird things when in a wheelchair and transferring.
MAKE SURE YOU ARE WORKING ON THE CORRECT LEG! (Yes, I did the wrong leg the first time. He had pinned them himself and was wearing them backwards. Sweatpants do weird things when there's only one leg in them.)
Step 2: Stitch the Ankle Fold
Step 3: Turn Leg
Step 4: Fold and Pin
Pin the top corners so they line up close to the side seams thru front layer only.
If the original leg was tapered, divide the excess and tuck towards the side seams. Pin the center back of the pocket "mouth" thru the front layer only of the main leg.
If needed this is a good point to have your customer test for fit. Be careful of the pins!
Step 5: Final Stitching
Stitching thru front layer only of main pant leg, stitch the center 3/4 of the back of the pocket mouth, backstitch start/finish as needed to reinforce.
Resist the desire to stitch all the way down the sides of the pocket. Resist the desire to lay the pocket perfectly flat and stitch all the way across the back of the pocket mouth. Both of those changes make the leg hang at a weird angle and/or make it look odd and stiff and have corners that stick straight up in the air when sitting. (The voice of experience.)
...on further review, my customer has no leg so the pocket tack points look best laying on the seams, pulls in the bulk and leaves a roomy pocket. If your customer has a partial leg, you may need to adjust the tack points to make sure the thigh isn't too tight when the pocket is full. Refer back to the previous step and have them test fit with pins.
Step 6: Enjoy!
Warning! Trying to carry to many heavy objects at one time may cause your pants to come down.