Have you thought of sealing the edges with wax? Using a softer wax, as opposed to a brittle one, should allow some 'give' so that the seal will not break when you pick up the container. I just know my grandmother used to seal canned goods with wax and it provided an air tight seal. Wax should be easier to find in your area and will be more cost efficient unless you are a big time producer and can buy shrink wrap in bulk.
Also, to reduce humidity in the flasks you can store them in your 'glove box' on bed of sterilized silica gel or with silica gel in small containers to remove the humidity. Once removed you will have to move fast to keep the flasks dry while you are getting ready to plant in them. You can reuse it buy drying it out in an oven.
I get medication with small plastic devices that are filled with silica gel, which reduces moisture in the bottles. I take these and heat them in the microwave, then place them in my fertilizer packages when cooled. This keeps the fertilizes from getting 'moist' from the humidity.
Just some thoughts to consider,
Steve
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