Properly set up sheet metal or wood screws are strong, but you are talking about mounting a heavy piece of equipment to a place that is bouncing and potentially flexing. I'd be worried that over time the screws could work lose. Through bolting through the bulkhead is by far the strongest mount. We have our inverter mounted in the lazerette bolted through the bulkhead wall that separates the head from the lazarette. Yes, there are the rounded bolt heads on the interior head bulkhead, but painted over they are not that noticeable.