SL1200G GAE Arm Board

I would think you can take the armboard off for a quickie measurement and get it back on in less than an hour of work.  Unless you are compelled to listen to LPs 24/7, that should be an acceptable amount of down time. (I am sitting here wishing I had time to crank up my system.)
After that, you can send the needed dimensions to Colby Lamb who lives near Portland, OR.  Colby can make what you want out of any metal you want.

If you are in the USA, then I can supply the name and contact information for a very skilled machinist with audiophile sympathies; he can make you anything you want out of metal.  But maybe I am misunderstanding your question, which seems to be about the needed thickness of the armboard.  One would assume that you own the turntable, else you would not be interested in a replacement arm board. If you own the turntable, then you are in the best position to determine the needed thickness, either by measuring the OEM arm board or extrapolating from a direct measurement thereof.  Ralph Karsten (Atma-sphere) also probably knows something about this.