I've used anything from big old coolers ($5 at hard sale) to big plastic storage tubs, depends on your budget, how big you need it and if you want it locking.
I have a 2' square, 18" tall one where I put a piece of HDPE or cutting board material across the top, so now it serves as a cutting board or emergency seat (if cleaned, lol!). Added a hasp and combo padlock to it - simple & works!
Go to any big box store near you or tractor supply store, as I bought a big 'grain feed' box that I use for a bail of Timothy hay for the rabbit and it's big enough to store many items and was < $25. This one isn't solid enough to serve as a seat, but would be awesome for lightweight/waterproof storage!