I didn't know there was a bigger SP cage! I thought this was the biggest. What model do you have?
And yesyes, details!

I got the carpet from Menards I believe. I don't remember how big their rolls were, but the cage is 20in deep, so I got a 2ft section and just cut them to size. I made two sets at a time from one section of carpet, and it costs about 4 bucks, so it's not bad.
To wash, I just toss it in the machine usually on gentle, and I use antibacterial dish soap and baking soda in it. The carpet doesn't have any padding on the back, just a plastic mesh stuff to hold it together, and eventually it starts to dissolve from repeated washing. But, that's usually about the time that the ratties start to nibble them, so I just toss them out and replace them.
Dried flat, they are thick enough just to lay flat in the trays. I tried to cut (heavy exacto knife works FAR better then scissors - I learned this after my second blister.

) them with a really close fit, so that helps. I've been toying with the idea of a bit of velcro in the corners though...
Also, in the summer, when the house is closed up with the a/c on, I sprinkle a little bit of baking soda on the shelves before I put the carpet down, and that really, really takes care of any little bit of smell from not having any fresh air circulating. I have a super sensitive sniffer, and most people usually don't smell them at all, but it works for me, and it's safe, so I figure why not?
I believe that answered all your questions and then some. lol