Here is a list of pretty much all you'll need to achieve this.
-slats, from the same bed if possible, to avoid differences, as it's quiet a precise job(as much as possible, depending on your box size)
-Krylon spray paint
-piece of steel
-line tape (masking tape) 3 and 5mm
Try to be as precise as you can. i've wasted a lot of time dealing with mistakes on drilling slats and steel, if you have space, try to drill both at the same time, it'll be a time saver (i live in a tiny flat so not a lot of tools and space :)
here we go
Step 1: Side panels
I'll explain the way to do it for one side panel only, you need 3 of them but the process is the same for each. So take 4 slats, or more, depending on your slats dimentions.
Cut your slats at the length of your box, the height for my box, and mark them with numbers, it'll help you ton put the right slat in right place when you'll take it appart. (40cm)
Put them next to each other, and use something to space them as much as you want. I've used screws to set the spaces, but evrythinng will do the trick. You need 6 spacers for a panel, i show you one end on the pictures, but there's 2 ends :)
Once your spacers in place, mesure the panel width. Cut to slats at this size, and pre-nail it (start nailing the above slat, in order to attache everything at the first hammer hit) like shown in the sketch, be precise, and 2 nails per slats to avoid twisting.
Put the pre-nailed slats in place, and hammer time!
i've used this way to do my box because of my lack of tools and place, the nails where usefull to keep everything in place will drilling 2 slats at a time,.
If you have used this technique, when you've nailed slats, mark the middle of each slats intersection and drill. Once more, do it for both ends (symetry)
Once drills done, take the panel appart, remove nails (tiny nails are easier to remove, just pull between 2 slats, et voilà)
At this step, you can scratck the wood where slats touch, in order for the glue to stick better and make the attachement stronger. Glue the top slats (with 4 holes) and put it back in place, use nuts in those holes. Screw and it will replace a woodman clamp. Clean the extra glue and let dry during the night