I have a 1993 Dakota Sport with an extended cab. I find that the things I put behind the front seats end up all over the place and I would like to use some kind of organiser or just some simple milk crates to keep things tidy. However, the area there just doesn't allow for that because of the rear footwells.

So I decided to build a floor extender. It's basically a very simple 'bench' that extends the area under the rear seats by about 9 inches.

Its all made from scrap I had lying around.
There's a board which is 53.5" (136 cm) x 9" (23 cm)
The centre "leg" is just a piece of 2x4. This should be 9" long but I just used a piece I had lying around.
The other 2 "legs" are 4" (10 cm)

and I covered it with some rubber material.

It means that the area behind the front seats is all one level plus the rubber material gives it some grip.