My neighbor told me to use the heaviest quilters template plastic that Joanns carries. They were out when I was there, but I found one piece at Hobby Lobby. Seems to be equal to what's in the Clover package and worked fine in the clamshells I've made.
If you've got the largest size clamshell, don't be surprised when you find twice the number of pieces you'd expect in your package. With that one size, you double the layers to make it really sturdy.
I was concerned about the stress on the thread holding the sides together, so used leftover hand quilting thread to sew them together...they seem quite sturdy.
The dollar stores down here in So. Texas don't have the plastic cutting mats, either
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