My dad and I built this one a little over a year ago: https://fordtraktor.shutterfly.com/pictures/191#190
I’ve added more limestone dust (built it up an inch or two) and interlocking mats. I wouldn’t change anything about it. I like having the taller inset posts because you can crosstie to those, then with young horses I put up the stall guards in the front to keep them quieter/make it feel more like a stall. It works great. also the inset posts ensure you are working on the mats and the horse isn’t half out of the crossties like they are if you tie right at the front.
Mine is 10 by 12 with the posts inset by 2 feet. I like the narrower width to keep the horses more still.
I like the mats over stone instead of concrete just fine and it was a lot cheaper. it drains well.