I bought a slim wardrobe type thing from B&Q a year ago now, it's ment for garden tools but is fab for horsey stuff. It's waterproof with a shelf and I keep my grooming kit, first aid kit, buckets, half bags of feed and rugs in it. I also attached sticky hooks to the insides of the doors for head collars and empty haynets. It takes up hardly any room and has a lock option if needed! I don't know whether there still available now though.
What about old chest freezers, take the seal off the lid tho. You should be able to pick them up for next to nothing. In the past I've used an old wardrobe, ok not good for outside.