It's been years ago when I did but I went online and it seems it was a DIY website that I found exactly what I needed but, I built one with my best friend. His son was a Pitcher so we actually built 2 of them.
We used pressured treated lumber just so that it would last longer even though it was heavier to move around. We did put handles on the front and back to make it easier to pick up. Im not a carpenter but, if you're going to be using pressure treated lumber because you leave it outside and need to move it around a good bit then Id recommend finding a way to put some wheels on it as suggested. That booger is heavy. Now if you plan on using it inside and leaving it inside then regular lumber will be fine and it wont be that heavy.
I used 2"x 10" since a regulation mound is 10" high. I think we had about a 18" or so flat surface on top and then simply went from the front of the landing down to about a 4" high on the front of the mound so that a standard 2x4 was used across the front. Then used some plywood for the surface so the indoor/outdoor carpet could be attached. I think we stapled ours down. We bough a rubber that had 2 pegs and simply drilled 2 holes on the flat surface so it could slide in and out. Very easy to make one of these.