Basic indoor setup:
I'm using CaCO3 with my misting water, as it promotes fruiting with many types, wood ash can also be used.
Any CO2 that dissolves into the outer water will convert to calcium bicarbonate.
This will trap the CO2 and also increase the pH of the inner bath.
Mist the propagator lid, and the cake, when its dry repeat.
Alternatively use a clear or white bag for the lid.