Down 25-11 after Tamou's try entering the final quarter, they rallied with halfback Lindsay Horrocks scoring a brilliant individual try, while winger Sikele Ulukuta landed his third penalty.
Those two were Border's go-to men on attack and cover defence, and while they weren't flawless, who could be with that much workrate.
Prop Jareth Hawea atoned for his first-half sinbinning for obstruction by scoring Border's first try to keep them in touch just after the break, while first-five Te Rau Wirihana looked much more assured late in the game as Border began stringing phases together.
In a tense finish, Border No 8 John Laurie, flanker Sheamus Murphy and Horrocks all had a close crack at the line, before calm Ratana second-five Vance Pereka snatched the intercept and booted his team clear of danger.
Ratana coach Johno Kaa was delighted to see the physicality return to his squad's play after the disappointment against Taihape and Pirates.
"Really thrilled. Our gameplan was to put our big pack on and bust them through the forwards.
"Being at home, that's a 10-point advantage right there, and we had that by halftime.
"Everyone in Wanganui knows Ratana will drive all day if we can - it's one thing knowing it, but another one stopping it."
Border coach Ross Williams was philosophical about the loss, which showed clearly that Border need some emergency strategies for games where they have been unable to impose their own will through key stages.
"Missing four from the pack didn't help us," Williams said.
"We won't be down about it.
"It's a season defining game. We've been winning ugly so it's nice to lose one ugly and work hard from here."
Border realised what kind of day they were in for when Rauhina ran over flanker Liam Richardson in the first of a series of big busts by himself and propping partner Kereti Tamou, while Border lock Jack Hodge also came to the sideline to get his bloody forehead strapped up.
Having to make a lot of tackles, the visitors still held Ratana until the 26th minute, when Rauhina marshalled his pack to lead him on a 20m drive from the lineout to score.
Border replied immediately through Ulukuta's boot, then Ratana countered through a Kane Tamou penalty, the halfback being the only successful one of three kickers his team would use.
Hawea being binned by referee Ben Lourie compounded Border's lack of weight up front and Ratana swooped with Paki scoring off another continuous maul.
Ulukuta closed the gap right on halftime, then the teams traded tries as Ratana hooker Dylan Marshall took his turn to be driven over by his mates, before Hawea got one back after a series of phases following Ratana spilling the kickoff.
It seemed Ratana reserve forward Charlie Mete had scored off a brilliant Tamou flick pass, but Lourie called it back for a 5m scrum so the Ratana halfback just dashed under the posts himself.
But Border would not lie down as Horrocks forced his way through a gap and stepped the cover tackles to roll himself over the line.
After another Ulukuta penalty, Horrocks and centre Sailosi Naqiso threw the ball everywhere in the dying minutes as Border raided Ratana's line, before Pereka cleared the decks and the Ratana pack kept Border trapped in their own 22m, forcing the clearing kick before winning the final lineout to hang on at the finish.
Grand Hotel Challenge Shield Harvey Round Motors Ratana: 25 (Vaan Rauhina, Kapene Paki, Dylan Marshall, Kane Tamou tries, Tamou pen, con) bt Waverley Harvesting Border: 19 (Jareth Hawea, Lindsay Horrocks tries, Sikele Ulukuta 3 pen). HT: 13-6.