Craig, I use these containers for my sunscreen and so far they have been leak proof. You can always double bag to be safe. They also come in different sizes, (i think up to an oz)...should work fine for butter. Of course then you're limited to only a small amount.
For olive oil, i try to use a pouch of some sort. I was able to find these aluminum pouche drinks that are very similar to the ol' platy lil nipper. I found mine at the local asian grocery store...works very well. Its something similar to this:
If you liquidied your butter, you could carry a ton in one of these pouches. The one I found was actually twice the size of the linked picture. It was also ergonomic and hr glass shaped lol. If the butter solidifies during your trip, you could probably just dip the pouch into your pot while your boiling water. If this is of interest to you, then you can easily acquire some. I think your from socal, and I know there's tons of asian grocery stores in OC