Here is our finished product!
No, I didn't choose the colors or do a lot of the painting.
But... we love it! It is sturdy and the boys can pound and screw into it, or on it, or even sit on it.
But how did we make it?
I found some scrap wood and cut sides to be even lengths with my circular saw and screwed them into the top (we did this step ages ago but the finished product wasn't all that sturdy).
Then we cut scrap 1x4 to be side bracing (you can see the first scraps are darker, pressure treated wood and the 1x4s are lighter wood) and screwed four pieces in.
Then my younger assistant chose the paint and went at it.