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
Step 2: Side Panels 2
You can remove nuts if you want. Take a slat and put it in place in order to complete the panel with a diagonal slat. Mark the lenght, cut, glue and use carpenter tool clamp. Let dry, sand it a first time, fill nails holes and the top slats joint (the "Z")
Sand again and again to make it as flat and smooth as possible. Now it's ready for design :)
paint it the color you want for your drawing. (white for this box)
Using line tape, make the design of your choice, and, when everything is ok for your 3 side panels, paint it with another color (black for me). Let dry, remove the masking tape et voilà!
You've got 3 painted side panels, you can try to paint the other side (the inside of the box), but for that you'll need some more masking tapes and tricks, i wont explain here.
Step 3: Assemble Side Panels
Step 4: Top and Bottom Panels
At this step you must deal with your "long screw". Look your panels from top, check corners, if the angle is not perfect, don't worry, the "proto screws" will do the trick. Cut it, and insert it from side to side, align your 3 panels angles, mark the good lenght, remove, cut, place and secure with nuts. Do that for each of the 4 "Screw deluxe" and your ready for top and bottom
Mesure the width of your box (the missing panel is the door), and draw as precisely as possible, full size, your top panel. (dealing with the inner width, depth of the box, the slats width, the spaces between, btw a bit smaller than the side panels, ...)
From your draw, design a temporary top panel from cardboard, to see if it fit nicely between the side panels. When it's ok you can cut your slats from your pieces of cardboard.
It'll look something like that, using the same process as side panels, nailing, drilling, taking appart, glue and nuts, 24h later tadaaaaaa!
Once again, as the side panels, sanding, filling holes and joint with wood filler, sanding, make it flat and smooth.
The painting process is also the same. First paint coat, line taping design, second color coat and let it dry. Remove line tape et voilà.
Step 5: Assembling the Box
When everything is in place and screw, it's quite a strong box!
Now you just have to deal with the door as a regular side panel, and find a cool way to close it, i'm still working on it right now, but i'll post my results asap.
Step 6: Finish
Here is a nice box to carry my bonsai arround :)
When i'm not using it for this purpose, i put it on its side and use it as a chest. With the nuts everywhere, it avoid scratches on panels