Greetings and welcome to RC Universe,
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In answer to your question(s):
Apply the bottom sheeting oversize and trim to the edge. Don't be concerned about the small gaps.

Fill the small gaps with baking soda and then apply a few drops of CA. This will form into something as hard as a rock so get the baking soda to the shape you need first. I use a small artist's pallet knife to place the powder.