Just using resin will not do sht. You have to use some matte or cloth along with the resin for it to have any strength. Dried resin is brittle and weak and can be easily cracked/broken without the matte and or cloth. If your just trying to seal it up for air leaks just use some silicone caulk. If you need the added strength go with the fiberglass reinforcement. Simple boxes that are well built, glued, stapled and or screwed dont need to be fiberglassed inside. Just a waste or time and money.