So here's the problem - a stimp of 10 literally means that the ball is rolling 10 feet off the stimpmeter. I have two pieces of automobile carpet on my basement floor just butted together but they are only 8' long apiece! I putt across the seam but figured it would skew the results too much.
You can find the dimensions of a stimpmeter online (I think its 36" but don't quote me) so I figured if you cut it in half technically is should work right?
So I made a (I think) 18" long stimpmeter out of a piece of "V" molding and balls rolled to about 5 1/2 feet. So I'm guessing my carpet rolls about 11 by my calculations.

Here's a guy who made one and its only 30" long.

www.jonathansantos.com/diy-golf-stimpmeter