The best method for calibration would be a bath filled with a chemical at its melting point with a solid/liquid composition. The constraints being that it must be hydrophobic and relatively non-corrosive and non-toxic with a melting point between (say) 20 °C and 60 °C. Needless to say, but I’ll say it anyway, the chemical must have a very precise melting point. Any ideas?
The standard point recommended in that range by the CIPM's ITS-90 calibration standard is the melting point of gallium at 29.7646 °C. Gallium is not known to be particularly toxic, but it can embrittle metals so is ideally used/stored in glass or plastic containers. It's used as a standard because it is relatively easy to make very pure and is stable in storage. Also, by picking an ITS-90 standard, you can ensure your thermometer is calibrated to the same standard as many other high precision thermometers. N.B. ITS-90 specifies specifically the melting point of gallium, not the freezing point, unlike some of the other calibration standards.
In the old days, you'd look it up in the CRC organic chemical tables, but now you can go to a site such as http://www.colby.edu/chemistry/cmp/cmp.html and enter the desired m.p. and constituents. Here is the result for a search for 50C (+-5C) m.p. without O, N and S so they're likely hydrophobic.
P-DICHLOROBENZENE PENTAMETHYLBENZENE 1-METHYL-4-PHENYLBENZENE 1,2,3-TRICHLOROBENZENE 1-ETHYL-4-PHENYLBENZENE 1,2-DIPHENYLETHANE 1,1-DIPHENYL-1-PROPENE CYCLOTETRADECANE 2-BROMONAPHTHALENE 1,2,3,4-TETRACHLOROBENZENE 1,2,3,5-TETRACHLOROBENZENE P-CHLOROIODOBENZENE
Taking P-DICHLOROBENZENE as an example, you would find it melts at 53.5 C and is mildly toxic, but is used in mothballs, so is a regularly available consumer item not requiring HM permitting to buy. See http://www.epa.gov/airtoxics/hlthef/dich-ben.html and http://en.wikipedia.org/wiki/1,4-Dichlorobenzene for more information.