When I’m not using the peels right away, I just leave them in their ziplock bag with the sugar and store them in a cool, dry place. Your refrigerator is not dry! Don’t put them in the fridge –
If using different citrus, you may need to add to the initial “boils” to remove the bitterness or to reduce the number of boils if fruit is sweeter, like an orange. Rule of thumb is 2 boils for oranges, 4 boils for grapefruit or limes.