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Polyfill Increases volume to the speaker not the other way around. This is well documented in all speaker and subwoofer design books. If your box is too big you Port it. If it's too small you polyfill it. Honestly the Best thing to do is polyfill and dynamat the inside of the box if it's too small this will increase the effective volume by up to 10%.
Download links on both knobs. but the faceplate has no download link.
All of the STL files are corrupted. Can you please re upload them or host them elsewhere and post the link?
Go all wood, then buy some epoxy paint paint it with a good epoxy paint and you will have a waterproof roof. If you really want super waterproof and make it last forever. get some fiberglass cloth and fiberglass resin. coat the roof in resin, lay down the cloth then saturate with resin again. if you double up the cloth on the corners the whole camper will last longer than the next 3 cars you buy. then after all the resin has cured, paint with a white epoxy paint.