Ceramic Heater Over Undercage Heater?

Yes, if you already have it. But plywood will be much cheaper if you have to buy it.

I'll be going for plywood then, at least until his permanent home is built. The temp has already increased to 86 on the warm side and seems to be staying consistent, all we did was cover the uncovered part (leaving about an inch for ventilation). I'll be monitoring it closely. Thank you so much for your advice

No problem. And you really don't need much space for ventilation.
The opening for the heater is plenty.