At 70 I believe you will find the room is not warm enough to keep your ambient temps up in a rack. Uth does very little for your ambient temps.

One option some have used is placing their racks or tubs in a closet and heating it with a small heater but I really doubt that would use less electric than your two current bulbs.

I think your best option would be to insulate three sides of your current tanks and make a cover for the screen to help keep heat and humidity in. You could use many different things like foam, cardboard, even thick paper. For the top I used cardboard cut to the desired shape with a cut out for the heat lamp to deliver its heat and light. I then wrapped it with aluminum foil. I just lay it on top so I can remove it when ever needed. Doing this may allow you to use lower wattage bulbs and will for sure help keep your heat in.

Ordering a better style cage and heat source will be several hundred dollars and that does not sound feasible for you right now.