Operating in the Philippines, we often carry fuel in plastic containers in the cargo bay of our Cessnas. The legality of doing so is questionable, however operationally it is a neccessity.
The company I sometimes rent from, strictly prohibits this. I'm unsure if this is in accordance to CAAP guidelines or company policy.
To be frank, if I was renting aircraft, I'd be unhappy if my clients flew with fuel in the cabin.
Best,
SSS