As to the thickness of the rubber between flask wall and pattern side surfaces. Rule-of-thumb is 1.5 times the minimum width. Assuming your sail is 1" thick, then the inside of the flask would measure 4". For the length, figure 1" in front, and 1" in back. Say your sail is 8" long, then the length of the flask, on the inside, would be 10". And allow 1" of rubber over the top and bottom of the sail master.
Us displacing blocks to minimize the amount of rubber needed, like this:
And use the flask as your strong-back during resin casting:
As to the very thin sectioned canvas shield: When you make the pour first subject the tool (and the resin within) to a hard vacuum to force out all the air that inevitably got trapped in the narrow tool cavities. Then quickly pressurize the work to at least one-atmosphere and let it stay there till the resin cures hard. Provide 'frothing' extensions to your sprue and vent network so you don't make a god-awful mess when pulling the vacuum.
David
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