You don't really need an oven to convert baking soda (NaHCO3) into soda (Na2CO3).  A clean metal pan and a cooktop or a heating plate will do just as well or even better.  Put a thin layer (less than a centimeter) of baking soda on the bottom of the pan and heat it strongly.  Stir while heating until the powder stops bubbling.  Don't use a pan coated with a teflon no-stick layer, because you need to heat the pan strongly and the heat could damage the teflon coating.

PS: I hope you don't mind me saying that I find your reports a bit chaotic, both the reporting and also what you report.  Perhaps you would get better results from your efforts if you put a little extra time in preparation and precision.