Skip the heating pad, won't do much for raising overall temp. It heats the bottom of cages for a hot spot for reptiles bellies, doesn't do much for the air.
I don't like the panel either, at least that brand.
Radiant heat panels from Reptile Basics would be a good option (smallest size offered should be more then enough), they are what I use in most of my Ball Python Cages. I also use Pro-Products Pro-Heat Panels, little more expensive and ordering is done through email so Reptile Basics is usually what I go with.
Issue with the panels is they need to mount directly to a hard surface, the whole back of it needs to be on a surface so there's no air space.
You're using a metal box but the panel would have to be mounted to the very top so it would be above your light.
Back to the ceramic heat emitter, it would be much safer in a metal box like you have.
You can use regular outlet style light dimmers to lower the heat or they can be controlled with on/off thermostats or grow controllers with outlet adapters. Same with the radiant heat panels.
Keep in mind that both ceramic heat emitter and radiant heat panels give off radiant heat, this means they heat objects directly and the heat from the objects heats the air.
I say keep this in mind because you will want to use an IR temp gun and makes sure the surface temp of the leaves closest to the emitter or panel is not to hot, air temp could be 70 but leaf temp could be 75 or even 80 depending on distance.