Substitute molten salt bath with temperature range from 400–500 °C
I wasn't sure what other conditions you needed for your salt bath, yet I listed few for your convenience together with the salt bath of $\ce{KNO3/NaNO2}$ (bath 5) for comparison:
$$
\begin{array}{ccc}
\text{Bath#} &amp;\text{Composition} &amp; \text{Approx. melting poing} &amp; \text{Working temp. range} \\\hline
1 &amp; \ce{NaCl}, \ce{KCl}, \ce{BaCl2}, \ce{CaCl2}  &amp; \pu{400 ^\circ C} &amp; \pu{500-800 ^\circ C} \\
 &amp; (10\!-\!15 \!:\!20\!-\!30 \!:\!40\!-\!50 \!:\!15\!-\!20) &amp;  &amp;  \\
2 &amp; \ce{NaCO3}\, (45\!-\!55), \ce{KCl}\, (55\!-\! 45)  &amp; \pu{450 ^\circ C} &amp; \pu{550-900 ^\circ C} \\
3 &amp; \ce{NaCl}\, (20), \ce{KCl}\, (30), \ce{BaCl2}\, (50)  &amp; \pu{540 ^\circ C} &amp; \pu{570-900 ^\circ C} \\
4 &amp; \ce{NaNO3}\, (100)  &amp; \pu{370 ^\circ C} &amp; \pu{400-600 ^\circ C} \\
5 &amp; \ce{KNO3}\, (50\!-\!60), \ce{NaNO2}\, (50\!-\!40) &amp; \pu{135 ^\circ C} &amp; \pu{160-550 ^\circ C} \\
\end{array}
$$

I also listed $\ce{NaNO3}$ bath in case you won't mind using it since it is not potassium salt and working range is ideal for your requirement. As indicated, you can change composition as much as you wanted to to get right melting point and the working temperature range you wanted to.