You could probably get away without a mockup, but doing one is usually safer.
I would personally use a lightweight twill rather than canvas. Canvas can be a bit thick and heavy. In period I guess it would be linen canvas. But it sounds like you are backing the linen with ticking, so that's probably enough on it's own!
For a draw cord, I would find some linen embroidery thread you like and use an electric drill to twist it into a plied cord. (Unless you have a spinning wheel, then use that.)
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You could probably get away without a mockup, but doing one is usually safer.
I would personally use a lightweight twill rather than canvas. Canvas can be a bit thick and heavy. In period I guess it would be linen canvas. But it sounds like you are backing the linen with ticking, so that's probably enough on it's own!
For a draw cord, I would find some linen embroidery thread you like and use an electric drill to twist it into a plied cord. (Unless you have a spinning wheel, then use that.)