I had seen (if my memory serves me correctly) small concrete (คอน/กรีต not คอ/นก/รีต) rollers that salt farmers dragged along to compact and smooth the pan clay surface -- that was yonky years ago. Later, perhaps some 10 years ago, I saw they used big graders and road compacters to do the same job. some 50 rais was prepared in one day including draining an collecting fish. But I didn't stay long enough to see how they collect the salt. ;-)