Odour can be a problem and a little embarrassing sometimes especially if you know someone else is waiting for the toilet after you. Unfortunately there is no way to prevent it other than to get to know the foods that produce a particularly bad odour, for example onions, garlic and cheese. However I love all of these foods and would not avoid them just for the sake of a smelly toilet. I do however carry with me in my handbag an odour spray (stoma nurse should be able to provide code to get it on prescription). In the UK the clothing shop New look also have pocket size perfumes which I quite like for £3. This leaves the toilet smelling much nicer and saves some embarrassment.
I've found a few things to work. There is a product called one drop that you put in the toilet before emptying. I have also used a travel size mouthwash and refill it so it's small to carry. I also buy cheap perfume, the smellier the better, on special at Chemist warehouse. I'm in Australia.