A PLYOBOX Construction, AKA Jump Box or Exercise Step Crate.

Here is my interpretation of the plyobox. A plyobox was known as a step box back in the 1980's. Some might call it a jump box or some other exercise related tool. The truth is , I have never used a plyobox to exercise. Inception Moment: But, my wife is very much into crossfit and uses a box to exercise with when she does a WOD. So, my wife asked me to build it in lieu of her buying such a box. She gave me the basic dimensions of the box and what her vision of it would look like and details of the use & abuse from crossfit it would have to be able to sustain. She started with the idea of a 12"x18"x20" -ish  sized box but I thought going big with a 30" width would make it very stable. And I was correct. Also , the hight and depth were increased a bit to 14"x20" to make it more challenging as she got stronger over time.    She thought of the idea of skateboard grip tape for extra traction as to prevent slippage off the box in a sweaty environment. I was going to try some sort of textured paint/ coating but the grip tape was a great idea. 

Materials: one 4' x 8' sheet of finished plywood 3/4" thick - 35$
                around 50 biscuits for biscuit joints - 5$
               around 20 drywall screws 2" - 2$
               sandpaper, acrylic paint, primer paint, one sheet of 11" x 58" grip tape-12$
               
Tools:  The usual  ... Table saw, drills, circular saw with plywood edger, drill bits, sander, biscuit joiner, jigsaw, paint rollers and lots of wood glue and #20 biscuits.  

I am an amateur woodworker, this is just a casual hobby to me. People with better skills and equipment have done other plyoboxes and this is one of many different ways to make such an exercise box. The final result is very strong, maybe a bit on the heavy side but that could be seen as a plus..hey it's for exercise..right?    My wife was very happy with the final design and uses the box often. My kids like stomping on it too. I too have used the box and feel it is a good workout. So , consider this design if you are contemplating making a plyobox. Good luck!

The first step is to plan out the cuts of wood that are needed to make the side walls. Factor in the actual width of the plywood, 3/4", to the final desired lengths of the sides. The one sheet of 4' x 8' plywood is plenty for a box of this size. 

Sides:     14" x 20"   2x
               20" x 28.5"  2x
               12.5" x 28.5"  2x

Two internal interlocking wall pieces : 28.5" x 12.5"   1x
                                                             18.5" x 13"     1x

These two pieces will have a slot cut in the middle so they can be interlocked together. There will be a dado type slot cut into the 28.5" x 20" sides, .25" deep to secure the cross support in place.    
               
The cut pieces need just a little sanding afterward. I used a Diablo  7.25" 60 tooth blade and it worked very well with just a few small side splinters on the cuts. 

Next, I put 3 biscuits on the short sides and 5 on the long sides. The trick with the biscuit slot cutter device is getting it set to cut exactly center in the side thickness of the wood. I use a scrap piece to practice with before cutting the pieces in the box. My slot cutter is a Harbor Freight one and was pretty cheap. It works well, is very loud. The setting screws that set the depth of the fence are very basic and they can move so they have to be tightened up very well. Once zeroed in, the tool works well ...enough.. The biscuits are # 20 size. A speed square is great for making the guide marks for biscuit cuts. The speed squares are good for holding the sides up at 90*, I used 2 for this while dry fitting the sides together.   The biscuit cutter can leave splinters around the cuts so sand down the area after cutting. I kinda lost count but it had to be over 50 biscuit cuts total. This step took me a few days to accomplish due to the nature of my life schedule. Maybe it could be done in a couple of hours if you can focus on it alone. Eventually , all the sides will have biscuit slots and you can dry fit the whole box.