Best thing for Rand to do is to withhold his endorsement and let the candidates come to him if they want it and I'm sure both Trump and Cruz want it, no reason to throw yourself behind either until the eventual nominee emerges at this point and that might not be either of them. Rand has a senate reelection to take care of before he'd get to work with any potential GOP president and it's probably a lot harder not having the support of the presidential nominee up ballot.