Heat Pumps and Electric Heating - how to safely heat a small concealed space?

I have a cabin that sits on posts in NH. no basement.
The only plumbing that is exposed to freezing runs inside an insulated box that is created inside the joists. I used to have forced air heat and a vent heating that box. I've since replaced the heat with a Rinnai direct vent heater for the cabin so I've lost that forced air heat source.
Is there a safe and reliable way to heat that box just so that the plumbing doesnt freeze?

The box is 10 feet x 4 feet x 8 inches tall.

At first I was thinking light bulbs or such.. but then ya gotta change em now and then etc.. Some kind of strip heater box with a thermostat would work.. but how would you figure the btu needed in the first place.

How about.. if you have access to the box from the main floor somewhere (or can extend it).. a supply and return grill.. and then some type of inline fan to keep the air moving. As it gets cold out, the box will just absorb more of the main cabin heat, and that heater can do its job. Probably good to put the fan on a thermostat control to save some power.

Go to your local box, hvac supply, or plumbing supply store and buy heat tape for the pipes.

Thanks, I had settled on heat tape as the solution, maybe running the heat tape in a conduit of some type (non combustible, so that if the heat tape failed, I could pull it out and fish a replacement back through.

Someone I know suggested just running the supply lines inside the walls and ceiling of the cabin (all are heated spaces) that would obviously be a lot more work up front, but would eliminate maintenance in the future. I'm going to investigate that as a solution first.

Thanks for all the suggestions!

This forum is fantastic!