Yes, as Mike confirmed, the dimensions I offered were made at the top side of the plinth. This is what the dust cover fits over in this design. There is a stopper on each side of the plinth near the front for the lid to rest on. With some other tables it may rest on top of the plinth itself.
At first I was going to measure the dust cover itself, but then I realize that might not be helpful. For one thing there is a slight taper in the sides of the cover, so it's topside is slightly smaller than the base. Also, such dimensions could vary depending on the thickness of the plastic used. So it seemed best to provide the dimensions the cover must fit over.
For height, that is the measurement of the inside of the cover. What I called "clearance to top of arm assembly" is the vertical dimension as my arm is set up for VTA, which the cover must clear when the arm is at rest. The difference of 1/2" simply gives margin for a higher setting of the arm if needed.
I appreciate you may be anxious to get on with this project but I agree with Mike, best to wait until you have the unit in hand. You can always cover it with a cloth once you have it set up to play, until such time as you have a cover fabricated. It will likely sound better with the cover off anyway.