Hackers Help: How to make the ÄPPLARÖ in waterproof/watertight outdoor storage bench?

Photo: IKEA.com

I love the small ÄPPLARÖ bench and am looking for outdoor bench storage for my cushions. My only issue is that it is not watertight. Any ideas on how to make it so? I am a DIY beginner but I’m somewhat handy with limited time. My thought was possibly putting some sort of plywood or plastic on the inside? Maybe plexiglass? The bench will be on my porch, but I live in New England and it’s not unusual to get blowing rain in the summer/early fall.