Of the several times I've had pastel de tres leches, only one of those times was I eating it in the same building where it was made -- so I think that attests to it's being portable. Of course, that depends on how far you're taking it, and how. I don't think it would survive a lone trip without refrigeration, but if it's no more than a couple of hours unrefrigerated, it should hold up well.
Alas, these were all made by friends or pastry shops or, in a couple of cases, made alongside friends, with their family recipes, which I was not able to obtain. So if you don't have a good Mexican cookbook, I'll hope someone else posts the recipe you need. All I can do is confirm portability.