i place the probe else were and see if you can get the inside temp higher (like between 2 layer fthe plastic (tub thickness ) so it takes longer to get up to temp then just adust it to the temp your tub needs.
some like this text refernce
probe
plastic
plastic
under tank heater
that and can placea piece of plastic above the probe to limit some heat lost .. but i would seriously keep a close eye on the tub temps until get stable.