Safety Question about making stainless steel flat-top
In my years of cooking in restaurants i do not remember seeing a stainless steel flattop. so i did some poking on the world wide inter tubes and found this quote from   


  wikipedia "The vast majority of commercial grade griddles are made from A36 steel, though some are stainless steel or composites of stainless and aluminum".


It goes on further in the wiki to confirm what you said about 304 stainless. 


  "Type 304 stainless steel is medical grade with a low level of porosity that reduces the penetration of cooking products into the surface, which causes sticking and flavor transfer – important performance metrics for commercial applications". 


So i do no know if there is a correct answer for the question. I keep a remnant of carbon steel 3/8 thick in my oven as a heat sink and under my pizza stone.

As suggested, try some remnants form work.

For your final choice you definitely want mass, commercial ones are 3/4" to 1" thick.
Go to a restaurant supply house and get a grill scraper and porous carbon scrubbing brick ( not sure if you use that on stainless ).

Oh the memories of standing over the hot flattop at the end of the night with oil and carbon brick scrubbing the day of the grill.