Do you also partial boil? If so you can try what i used to do with great success which is to use frozen blocks of sterilised water as the top up "water". You can use large stainless steel bowls and foil covers or large plastic containers with lids. Just remember that water volume expands ~10% when frozen.
If you do full boils, you can try what I do now which is to lower large PET bottles of frozen water into the hot wort. Just make sure the bottle walls are thick (typically for carbonated drinks) and not the simple thin type used for distilled water as they crack in the heat. Of course, you should sanitise the bottle surfaces thoroughly after removing from the freezer - especially the subsequent rounds of bottles used when the wort is no longer boiling.
