Building your own box wouldn't be too difficult quite a few builder's merchant's have a cutting service so you can get the main panels cut ready to put together. Using plywood would give you the nice timber finish and hide the laminated edges with a trim probably worth a visit to your local shop to Read more… get some idea's. Also a Google and YouTube there are way's of using magnets as hidden lock's to keep it all well from any accidental discovery's lol

For shopping I usually check out honour it's now skin2 they have a really good range and are pretty good on price it's worth checking amazon aswell as eBay...for storage just use an old suitcase you could try a charity shop or antique shop for an ottoman

As to my earlier post on the thread you can lay a sheet of plywood over the slats to spread the load 😉 you can also get rings on plates you can screw to it near the edge or better bolt through and put a backing plate the other side no matter how much they wriggle they ain't getting away 😀

8x4 sheet of plywood 12mm should be thick enough but you can use 18mm if you think you may need it lol ...work in a builder's merchant's there are so many thing's with a little imagination can be used as stingy toy's lol