I've used the yellow Heet bottles without any issues. I didn't break the factory seal until I was ready to use some and after that it was packed where it was kept upright when the bike was upright. I never noticed any leakage onto my other gear even though the bike was laid on it's side sometimes.

That said, you might try making a better gasket for the yellow Heet bottle cap. That or finding a better cap that fits the yellow Heet bottle.

Way back in the day I have used pop bottles. They work great other than the risk of being mistaken for a beverage. I drew skulls and crossbones all over them, but I still don't recommend that as sufficient. Personally I might consider a combination of labeling and keeping it packed away where you are the only one who has access to it. If going that route get it out only for use and pack it away as soon as done with it. Probably better to use a real fuel bottle though.

Edit to add:
I was checking on something I had read about a custom spout cap for the yellow Heet bottle and read that the yellow Heet cap changed sometime in the last several years. I had a bunch of bottles in stock and hadn't bought any on the road in some time and if I had recently they might have been old stock. So bottom line, I probably have no experience with the current cap that is on the yellow Heet bottles and my comment may be based on an old cap design.