I decided to whip one up similar to the one i have. It has worked great and the design is good.
First i decided the size i went with, it is roughly 10 inches by 5. Deciding how to make the inside, i cut it out 11 by 10 inches. So then i could fold lengthwise to cut down on hassle.
I used a short stitch length and went over it twice, then went over it again with a zigzag. Once would probably be enough for normal use, I decided to make mine stronger because Munch has a habit of playing rough with mine.
I stitched up one short side, the long side and then just around the corner so it would be easier to finish off later.
I made a rough funnel out of junk mail because Munch had disappeared with mine. Inside is rice, about 2.5 cups. I really filled it to how much i like it.
I took it back to the sewing machine and went over the opening twice with a short stitch length and then again with the zigzag. It is awkward to do it without the rice in the end, so just go slowly.
I took it back to my 'workspace' (read: floor in front of heater) and worked out how i wanted to assemble the cover. I decided if i cut it a few inches wider then the insert i could play around with fit. I just cut it the whole length of the fabric to make it easier.
Next I set the strip out of the floor with the insert on top. I fiddled around and decided how much i wanted the top to fold over and how far under i wanted the bottom side. I think i have an overlap of an inch or two. I marked it and then did a hem there.
You can sort of see how if it was folded properly, it would overlap. The side on the left will be the 'top' or 'outside flap'.
I then folded it as i wanted it inside out and sew up one side. Took it back to my workspace and worked out where the line for the other side needed to be so i could fit the insert in nicely. Once i had that decided, i went off to the machine and stitched that up.
Hard to see, but one side is stitched up here.
Then i trimmed the seams and turned it the right way out. I found shoving the insert in quite easy and folded the top over. It fit perfectly, length and width even taking all the rice into account.
Here you can see it compared to the old one and all finished.
I usually heat the old one up for 4 minutes in the microwave. This one, i popped in for 3 minutes and it is toasty hot, the old one, is just warm.
Also, my old one has instructions to have a mug with some water in it when you microwave it. I usually put in about an inch of water. I am doing the same with my new pack.
I used it overnight, although my old one keeps the heat for longer, its not so much longer that i would chase down a different filling for them. The rice is easier to heat up and therefore, i get warmer sooner.