Many years ago I used to use sealable clear polystyrene plastic "cake boxes" about 25cm x 25cm x 10cm. You have to use one with an overlapping lid. We would put a fixed amt of baking soda / baking powder in a small container/beaker in the box and put plates around it. Water would then be added to the container and the box very quickly sealed with insulation tape and placed in an ordinary 37C incubator. You may wish to use a HEPES buffered media. We kept cultures (fibroblasts) going for 2 weeks until colonies were visible.

Thanks Teo, your answer has given me a direction. In addition to your suggestion, i am also considering a system that is totally devoid of incubator. A sstem that may be placed under room temperature while maintaining 38-39oC, and 5% Co2 internal microenvironment necessary for IVC of porcine oocyte.

You would still need a heat source capable of maintaining a constant 37-39C. Short of an electric blanket with a thermostat I cant think of anything off-hand