From my experience, any citrus peel will help. Years ago when there was a bad flu and a friend in Brazil (she was American and married a Brazilian) emailed me that she was very sick, her hub was in the city, and what should she do. I asked if she had access to any citrus; she had a yard full of lime trees. I told her to chop a bunch of peels into strips, cover with a good amount of water, simmer for about 20-30 minutes with lid on, strain, and drink 1/2 c or so with some juice and honey. Cured her very quickly. So even lime or lemon will do, and orange is also strong. Grapefruit would be of course good if you can get any.
I like the idea of frozen cubes, I think I’ll try to make some if I can get some organic citrus (the only kind I eat anyway, the put serious poison on citrus).
I just remembered another thing that can be helpful - when feeling sick, drink a small cup of warm to hot water with 1 or 2 drops of pure lemon essential oil (could be grapefruit EO) and a spoon of honey. Stir while drinking as the EO floats on the top, or could mix the EO and honey, then add water. Tastes good and and 3-4 cups a day would be fine.
Where do you find organic citrus fruit? I’ve never seen it, and we can pretty well find organic just about anything else in the supermarkets.