What I have seen done is you get a 3/4" thick plywood, half sheet, or whole sheet, then use a hammer that shoots 22 caliber load to drive a nail through the plywood and into the concrete. HomeDepot sells a Ramset brand hammer. You would shoot a nail into each corner of the plywood, maybe some along top and bottom edge, enough to hold it. Then you can use screws to mount the inverter bracket to the plywood. Remember to use safety googles.