Saran wrap will work. Typical poly trash bags work. Use light poly or saran since they will conform easier (than heavy poly) to any compound curves. Mark the centerline on the part before you pop it off the plug boat. Don't rush to increase cockpit size! But, if you have to, the hose + cardboard will work. You will end up holding the forms in place with masking tape. A thick coat of car wax should enable you to peel out your temporary forms. Build it to a stock size...or build a custom spayskirt next.
Test your wax / epoxy release first...just in case. A few drops of mixed resin is all you will need for the test.