Find a piece of thin-wall PVC tubing around 2" in diameter. We used a "drain tube", designed to connect a sink drain to a trap. It was a few dollars at the local hardware store.
Attach it to a convenient location using self-adhesive Velcro.
Stuff it with "T-Shirt" grocery bags. Really jam 'em in there.
Presto! You have a Plastic Tube Trash
Bag Holder. When you need a bag, just pull one out of either end. If you
can't reach the next bag to pull it out, use a spoon handle or other suitable
rod-like device to push the remaining bags closer to one of the tube's
ends.
The bags compress nicely when they
are stuffed in the tube. Here's one that I just pulled out, next to a quarter.
Here is the same bag and quarter, with
the bag opened up.