There's a possibility the MOSFET wouldn't be needed at 5V. When I've tested this with a PNP transistor, I found that applying reverse polarity produced no current - up to 8V. So it acted like a Zener. You could test this with your meter. Apply reverse voltage through an ammeter, and see at what voltage current begins to flow.
Also, is there actually a risk of reverse polarity here? The risk usually comes with batteries, but you said 5V, which doesn't match most battery types.