Hmm I thought I already posted this, but here goes.

I wasn't able to find any comparable material amongst my supplies, so I went to Joann's fabric and got a few yards of this stuff for around 80 cents/yd.



It looks like the right stuff, if not, it'll still do the job nicely.
It's cotton and dye-able to match tartan colors. It's going to be a lot of work to get all of them sewn on, but much preferable to the alternative. :-)