I’ve tried the Nathan flasks*. They are ok but if the gel is thick flow is not so good. Plus, I could really only get about 2/3rds of the contents squeezed out ever.
I tried just re-using the EFS liquidshot ** flasks. I thought they worked well even for a thick gel but they don’t hold too much & really aren’t made for re-use. Not a bad option. If the gel is thick, you can’t really get more than 75% of it out of the flask.
I tried these off-brand flexible flasks*** & they would be just about perfect except you can barely get a SIS gel to flow through the opening. It will flow but you gotta really squeeze it. Actually I remember you have a pretty good grip so it might not be a problem for you. Or you might just destroy it when you sqeeze! So be careful if you purchase. Good capacity on these & you can get out almost every last drop. (I’m not kidding about the grip thing. These are solid but they won’t stand up to a real solid grip. You’ll make a mess and be out of calories.)
These flasks from PlanetX might be my favorite****. Cheap. Reusable. Good flow. Who cares if you drop one. Only downside is not much capacity.
*https://www.amazon.com/NATHAN-Flask-Cap-Pack-8-Ounce-Silver/dp/B009T424CU/ref=sr_1_1?keywords=Nathan+Flask+with+Race+Cap-Pack+of+2&qid=1566156719&s=sporting-goods&sr=1-1
**https://www.amazon.com/Endurance-EFS-Liquid-Shot-Vanilla/dp/B001RR5EFG/ref=sr_1_fkmr1_1?keywords=1st+Endurance+EFS+Liquid+Shot+Flask+Vanilla%2C+5+oz&qid=1566156902&s=gateway&sr=8-1-fkmr1
***https://www.amazon.com/TRIWONDER-Folding-BPA-Free-Collapsible-Hydration/dp/B06XRWF1DR/ref=sr_1_6?keywords=TRIWONDER&qid=1566157126&s=sporting-goods&sr=1-6
****https://www.planetx.co.uk/i/q/BOGELFL/gel-flask-bottle