Update, it works! Actually it is better than a foot rest (I used my backpack for a footrest on the plane) because it supports the knee as well. My right knee gets sore if I don't move it, old skiing injury. I can still ski but riding a chair lift is painful because my knee has to hold the ski weight.
I am short and sometimes my feet don't reach the floor on airplanes especially when I am reclining. Most planes don't have footrests on economy so I decide to make my own. I used belting material, a belt buckle that adjusts and the bottom of a shopping bag. Tried it in the car and works great, can't wait to get on a plane and give it a go.
