Here's what I ended up building:
- 3 - 2x6x10ft treated lumber
- 1 - 2x6.x8ft treated lumber
- 1 - 2x4x8ft treated lumber
- 1 - 4x8x1/2 treated plywood
- 1 - box 1 1/4" coated deck screens
- 1 - box 3" coated deck screws
- 1 - pitching rubber (Amazon)
- 7 - 1/2" reversible foam mats
- 1 - tube foam adhesive
The dimensions of the mound are 84" x 42". The end with the pitching rubber is the full width of a 2x6 (~5 1/2" ) and it tapers down to 2" at the bottom. I choose that height because in little league the mound is 6" above home plate. The flat section with the rubber is 22" inches wide before it begins to taper.
I cut away a section from the sides and back of each board to help reduce the weight and to also give the grass underneath the mound a chance to breathe. It also makes it much easier to lift up if you need to move it. I plan add some portable wheels eventually, but I'll worry about that this spring when it's time to move it :)
Here are a few more pictures from different angles:
You can see from the picture below that I set the bottom of the plywood 7/8" of an inch below the top of the 2x6's. That way when I added the 1/2" foam it would be roughly flush with the top. The pitching rubber is 3/4" of an inch thick so it sits above the foam slightly. I debated on adding an extra piece of wood below the rubber to raise it up a bit more, but I figured that the foam would compress some after use.
The reason that I used foam rubber tiles is because I had a bunch of them laying in my garage collecting dust. I considered indoor/outdoor carpet, but I thought the kids would appreciate having a bit more cushion. I also seriously consisted buying a cheap rubber stall mat, but the mat alone was close to 70 pounds.
He's an angle from the bottom and as you can see it's not very pretty. If I had had some extra 2x4's laying around I would have built a 1 1/2" continuous frame around the perimeter to support the plywood. Instead I simply used the scrape pieces that I had left over from side and back cutouts. It seems to be plenty stable for the kids, so it worked out OK. You can't tell from the pictures, but I also drilled some drainage holes at the bottom of the mound.
Hopefully someone will find this useful. We'll see you in Williamsport (go Coon Rapids Andover American Little League!)