I've had a quick look at that post and all I can say is WHOW, You must trust others more than some other people and rely on a posted picture which was posted 10 years ago with little or no information about it and only one person say's I've built and it works,
The idea is there's one for high side and one for low side.
Your trying to use the LOW side in HIGH side you need move it to low side so that no ground (-) get's fed into the board if connected wrong. The OP could not even be bothered to reply to that thread. And I'm not sure why his using to mosfet's in series for either pointless.
Why don't you just use a signal P channel Mosfet it's doing the same job with no voltage loss like the one I posted. If you are getting voltage drop then your not fully turning the mosfet on.