Question about Keter Ontario

Hi, Quick question, as the box will be exposed to all the weather in the garden. Is it 100% waterproof or do you get wet lounge cushions after every big storm? I'm asking because I've already had a bad experience with a "waterproof" box, which was about 20% "waterproof"... could have been used more as a pool after the rain, as the water got in but couldn't get out again. My largest cushion is about 125 x 79 cm, does that fit? I only see the outside dimensions... Kind regards and thank you