I use a 'drybag' available from outdoor shops. They come in loads of sizes, I have one that I can squash about 4 nappies in, then you squeeze out the air and roll the top and clip it. No leaks or smell on a hot day. I then give mine a good rinse out at the end of a day out. They're cheaper than a nappy wet bag and I find the roll top more secure and easier than a drawstring.
That said I maybe over use dry bags, as I use one clipped to the handlebars of the buggy as a hand bag on rainy days!