Wait, you are using a heat belt to warm a bottle inside a power cooled chamber?
And you want to keep the air temp A but have the beer temp >A
That just seems counter productive to me.
I currently have my ferm fridges set with 2-stage controllers and it basically works like so
I set the temp for "50*"
If the probe is <"49*" plug 1 activates (which is a heating pad attached to the back wall)
if the probe is >"51*" plug 2 activates (which is the fridge itself)
Plug 1 should never be on while plug 2 is active as the probe temp should never be both above 51 and below 49.
That make sense?