Wanganui's tryscorers included lock Jack Hodges getting a double for the second game in a row, while other 2019 Steelform Wanganui Heartland players in winger Karl Pascoe and loose forward Chris Breuer also dotted down again.
Young winger Tawera Puohotaua snatched an intercept try under the posts when Horowhenua-Kapiti tried running it out from their own 22m.
Other standouts were hooker Jack Yarrall and loose forward Bradley Graham, who took over the captaincy of the team after Jamie Hughes was called up into the Heartland squad in Week 2.
While the team did not have a great preseason with losses to Hawke's Bay Under 19 and the full Wellington Maori side, they dominated RDO Shield with double wins over the Wairarapa Bush and Horowhenua Kapiti Development teams, while Wellington Maori Development withdrew from the competition.
"It's been an awesome bunch of boys, with an awesome culture," said McNaught on his first season in charge.
"Pretty easy when you turn up, get a gameplan, and they play to it, with some good attacking rugby.
"Hopefully guys like Chris Breuer get another chance [in Heartland], and Joe Edwards and KP [Pascoe]."
With both the Development XV in Levin and the Heartland side away in Oamaru, Whanganui Māori requested a postponement for their Saturday game with Manawatu Māori for the Miki Haddon Memorial taonga at Feilding's Johnston Park – due to not having enough players.