That area at the back of the hard bags is a nuisance because of its shape. I've stuffed wet weather gear in there and it falls out before I can get the liner bags in. So I came up with this.
It's a panel made from the cover of a plastic pocket folder. There are two hinges in it, one along the bottom to allow it to fold forward and one oblique one to allow it to sit in the narrow floor of the bag.
This is in the closed position.
This is the open position.
With a rain jacket loosely stuffed in.
This is what the piece looks like, showing the taped hinges and velcro positions. The little appendix piece at the bottom right has since been cut off as per the drawing.
A PDF of the piece with dimensions is attached.
To make the cover, cut two pieces to the dimensions shown, mirror images if your plastic sheet is a different texture on each side.
Cut the small panel off the bottom.
Cut along the angled line.
The hinges are made with heavy tape. The first side to tape on the lower hinge is the side facing into the bag. Make sure the pieces are closely fitted together. Then fold the hinge over with the tape to the inside. Now tape the outside with the tape going along the outside of the fold. You can now fold the piece from vertical to the front.
With the oblique fold, first tape the side facing into the rear pocket, once again with the pieces closely fitted together. Now fold the hinge with the tape inside. Tape the other side, once again folding the tape over the hinge. This piece now folds up along the side of the bag when the panel is opened.
I used sticky-back Velcro along the bottom to hold the panel in place, and again at the top to close it.
From Woodrising (no, nobody else has heard of it either)
Rides a Springfield Dark Horse