I've never done this - but what I would try is to give yourself more time than you need. Figuring an hour per pound, if it's a three pound tri-tip, give yourself three hours.
Cook it with a temperature probe in it and when it comes up to your target of 135F, pull it and wrap it in foil. Then you can place it in the center of a cooler packed with crumpled up newspapers to hold until serving time.
That's what I'd try anyway. I'm sure there are others who have actual numbers for you around here somewhere.