I use a FOAM puzzle fake wood floor. 

It is very easy on the knees, and keeps the floor from being cold on
those fall/spring mornings. 

Looks like this and has held up very well. Though you do need to watch
for dog claws & firewood damage.
http://i22.ebayimg.com/01/i/000/d2/e9/4456_1.JPG
Ebay Auction #: 160258958774
I bought mine locally @ a closeout store, but on ebay it's $5 for 8sqft.

I have to point out that it's FOAM and SOFT hardwood to people looking
at my van, they don't tend to notice right away, or from ~3+ ft.

It does buckle a little in the summer heat (expansion) I have it cut to
tight to the edges, needs a little more breathing room. Also stuff
dropped on it doesn't crash or break.. And it wipes clean easily. Does
not absorb water. 

Dan in NY

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From:    B <oddstray@ODDSTRAY.COM>
Subject: 

Hi,

Here's a glimpse at our passenger-area flooring:

http://www.sdmcriders.org/Oddstray/westy/screen/lower1.html

There are nine sections of 1" thick foam flooring.  Sold with 'train
town' image at Home Depot.  Easily cut to fit the floor of the Westy.
Easy to clean.  Easy to remove whenever we might need to.  Cheerful to
look at (though, we do cover ours with an additional layer of indoor/
outdoor carpeting because it makes B's old knees less cranky when she
kneels on it).

B&S
'87 Westy 'Esmerelda Blanc'
SoCal