Sugar water with some cordial and salt is hard to beat in a gel flask - cheap, efficient to make and tastes OK. At the ratio I make it with, it has the consistency of thin syrup.
To make enough for 4 hours of riding @ 80g carb an hour, the night before, I put the kettle on and weigh out the sugar in a jug: 320g granulated sugar for 2 x 250ml flasks. I add salt to requirement and pour the boiling water from the kettle to the 500ml line on the jug or 500g on the scales, stir and add cordial to taste. Doesn’t need to be fully dissolved, put it in the fridge overnight, in the morning decant into the flasks.
They fit in 1 jersey pocket and I drink half a flask an hour. It also means that you’re technically carrying an additional 500ml of water in the flasks, giving you more time between stops.
I’m trying these flasks at the moment: