I found that Jim Carter's kit was woefully inadequate - painfully (on the butt & back) inadequate.

It might have been the loose weave burlap. Jon G's burlap looks much better.
Also, JC's other layers are thinner than the ones in the sales illustration, above.

JC's seat cover installation kit
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JC seat cover installatiiion kit.jpg (26.72 KB, 254 downloads)