Oh, I thought this thread would be about clothing, and then the obvious answer would be to gain a couple of pounds. Squats, deadlifts and all that..
Another idea is to put the kit bottom down on a piece of cloth, draw the bean shaped outline with a centimeter or two to spare. Then measure the length of the shape with a string. Make a sleeve with the same width as the length, make it 15 cm higher than the cooking kit. Sew it altogether and end the sleeve with a drawcord. That's what I would do if I already didn't have a great bag.