CHE will work day or night. In a 120 I would use three smaller CHE instead of one large one. Make a top or use foil to cover most of the screen top. So the top would be solid with O.....O....O. that would be if the CHE is on top. If you can find the right safety cage for a CHE you can hang it inside the cage an cover the top with R-Max foam board. Place a large water bowl under one of the CHE's. That will help a bunch. I'm slowly replacing all my glass with Animal Plastics. An ran CHE or heat lamps for years (1970's).
Any Reptiles shows in the Great White North near you? Somebody has to be making or carrying PVC cages. Or a sheet of PVC an a router an have at it.
Currently it seems that only Cornel's World still has stock. I've tried contacting PVC Cages by phone and email as I am looking to upgrade but haven't gotten any response so they may be inactive due to Covid. Allreptiles is out of all except 24"x24"x12" PVC cages and that is too small for a BP.
Cost efficient would depend on:
1) Your skills.
2) Long term use. PVC in the long run is cheaper then glass (heating). Up front (an ten years or so) glass wins.
I have glass an Animal Plastics. My T8's replace 125-175 tanks. PVC is much easier to clean, heat an get into. Heat panels outlast 1,000 CHE an cost less to run.
Glass works, it has for 100 years. It's just more work. In the dark cold Great White North I would think a glass an plywood tank would work well an still be cheap. Not much skill is needed to cut 4x8 sheets an screw them together. OK, some skill is needed to make it look clean an straight. A free 20GL tank would give you front glass.
If you search the forums you'll see some home made setups that are very nice. Same with glass an all the tricks to keep the heat in (foam board insulation).
Everything takes time an money. Only you will know what's right for you an your pet. Me, if I hit power ball for 90 mil, I building a reptile zoo (someplace warm)