Does it take kindly to folding, or should it be stored rolled up?
Rolls up just fine (wouldn't think of folding it) around a six-foot length of 1 1/2" dowel. A similar length of plastic pipe would be perfect, too.
The bonus is that the mat stores standing on end, so it takes up very little room.
That's really cool. Did you do the base layer of caulk and texturing at the same time or was it a couple of layers?
We used almond coloured caulking with a couple of tubes of grey and brown thrown in to create a medium toned grey-brown. Then we mixed in some water and some left-over paint wash (perfect for adjusting the hue if it isn't where you want it) and when the mix was creamy and smooth, but held stiff-peaks, we mixed in the fine sand.
The finer the sand, the better your schmoo will spread and the easier it will be to go back and add more schmoo over top without it looking obvious. A good thin layer, done right, will hide the weave of the canvas, provide plenty of texture for painting and will remain flexible. Your mat will weigh less and will cost less to make.
Pro Tip: Don't use a caulking gun! Have a container of water handy, put on the disposable gloves and get your buddy to cut the tube of caulking open from end to end. Spread the tube open and with
wet gloves, scoop the caulking into a bucket. Keep your gloved hands wet and emptying those tubes will be an easy job.
Definitely plan to do a mat and at minimum one hill or other feature with the same schmoo and paint treatment, so use a big bucket and make a lot. We used eleven tubes of caulking and had about 2 litres of schmoo left over after doing the 6x9 and the big hill. I still need to do 9' of railway embankment and would like to do a test of a patch of grassy ground. I think I have enough..

Left over schmoo should keep if kept in an air-tight pot or jar. But we have yet to keep any around to find out for how long.