The trackball mounts in just under the amount of space needed for mounting plate. While it has a diamond or football shape it takes up less space than the mounting plate. Looking at the shape of it you'd assume it sticks out past the square dimensions of the mounting plate, but it doesn't.
I used a boltless 3" TB mounting plate from Happs. That worked fine. If you need the exact dimensions Happs has a mounting diagram on their site. The TB is heavy, but not so much so that there is a problem with mounting. The mounting plate will hold it solid without any problems.
Home Depot does sell 1/2 inch MDF, they only sell it in 2'x4' sheets though, not the bigger 4'x8' sheets. In the lumber department they have a "pre-cut" section where they have the MDF, particle board and malaname (sp?) in the smaller sizes. You'll find it there for $4 - $5 a sheet.