I use something similar, but have found that using a slightly larger container than the GU flask (5.5oz/167ml) allows me to get a mixture (I use INFINIT Nutrition) that is not quite as thick as a gel, allowing for easier consumption, greater consumption (ie easier to eat everything in the flask), and easier cleaning.

I really like Hydrapak Softflasks. In fact, Hydrapak is the producer of most of the flasks and even water reservoirs in the industry, a lot just get designed or rebranded by other companies. I started off using the 500ml Ultraflask, but the shape of it (long and narrow) is not conducive to staying in a jersey pocket very well. I imagine the 300ml Ultraflask would be better for this purpose.

However, I’ve recently switched to mixing my liquid nutrition directly in my water bottle. For longer rides I typically carry 1 bottle of INFINIT, usually with 2 - 3 hours worth of formula, and 1 bottle of water. The INFINIT is isotonic and not very thick, so it doesn’t require being chased by additional water. As a result, I find that this set up is good for about 2.5, and up to 3, hours of my nutrition and hydration needs. In the summer, I might need more water and will resort two full bottles of water on the bike and a third with nutrition in my back pocket.