I have the Porta Pot, but it's the single most awful thing about our boat. Any kind of sewerage holding is bad, but having to empty the porta pot is pretty gross. It's also a trick to keep the smell down. Lots of good info about regarding keeping the smell down, from a handful of dirt to various bottles of pink and blue nuclear waste.
Composting toilets are the way to go, but you need to use them reasonably often. Not much smell, but they don't fit well in the forward v-berth. So I've stuck with the Porta Pot, but wish there was another solution. One sailor I've been on a two day sail with didn't actually do #2 for the full two days.... You know who you are.....
So that's another method of storage, just keep it in....
There are solutions for plastic holding bladders, that you can pump out, but space for fitting one is limited to either above your head in the v-berth, or below one of the benches/sea berths in the main cabin. Unless you use some of the space in the stern under the cabin, but then you have a lot of plumbing to go south....