Per Koji's suggestion, I added two terms to the expression for test mass cooling power for radiative/conductive cooling from 295K (see 2641):

;  .

I added 0 as a lower bound for fitting a, b, c, and d. The best fit for d came out to be neglible (on the order of 10^-17): conductive heating from room temperature can be ignored. The best-fit parameters for [a, b, c] came out to:  [0.025, 0.003, 0.001]. The result can be seen in Attachment 1. 

This fit would imply that there is some radiative heating of the test mass from room temperature, in addition to expected radiative/conductive cooling from the inner shield / coldplate. It is worth investigating what might be causing this, and if nothing can be determined then the model needs to be revisited.