how about getting a large container and filling it with very hot water ... and make a water jacket
to raise the temperature of your dektol ... if you don't have something large enough for your gallon of dektol
pour off some of it into a smaller container ( like a hand tank for processing film ), and put that smaller container in your hot water and stir until the powder is dissolved.
then do this again and again until you have remixed all of your dektol.
i have found using metal tanks for processing 35mm ( &c ) film transfers heat readily to whatever liquid might be inside of it.
good luck !