I still think my initial suggestion of a second UTH at a lower temperature than the hot side would work best. But I am fairly certain the RHP will keep your humidity higher than a CHE. With CHEs they focus the heat toward a spot, and that decreases the humidity there, which gradually expands to around that area.

Now RHP will do the same thing, but to a lesser extent, heating up the air, then heating up the substrate, then reducing humidity. A similar effect can be managed with a CHE by focusing it on one of the walls of the enclosure, from a distance, instead of on a spot on the substrate. This will heat the wall, which in turn will radiate heat the the enclosure (and depending on your wattage potentially do all kinds of nasty things, which can be avoided with distance and regular temperature checks on the wall, until you know how it increases in the long run).

Now with an UTH it would heat the more moist substrate on the bottom, which should get the humidity trapped somewhat within the substrate above.

Honestly, if the roof of your enclosure is solid, I wouldn't even suggest changing the heating anymore with that design. Reduce airflow and swap to a better substrate (aspen is a humidity nightmare, which is why I use it in my tubs, I don't need the humidity to increase beyond what newspaper or aspen will do for me).

So, cancel the order, restrict your airflow, and get something like reptichip. Your humidity should increase heavily