Heating bottles while out

When you are out & about w/ baby, how do you warm up the bottle? I have tried a few different ways but I was wondering what works best for you.
Do you keep bottles in a cooler pack?

I've seen bottle warmers that you can plug into the car cig. lighter. This is an option when you have no access to electricity:
Portable bottle warmer

The reason I asked about whether it is formula or BM is because my Luke was formula fed and DH and I learned a really great way to make it. We used the powdered formula and would make his bottles with 1/2 cold water, 1/2 boiling water. It worked really great. It was always the perfect temperature. So when I went out I carried the formula dispenser with the 3 compartments and a 1/2 bottle of cold water. Then I would get the boiled water at a restaurant or whoever's house I was at. If I was going to be somewhere that I wasnt going to have access to water, I would just put a unrefrigerated bottle of bottled water in the diaper bag and make the formula using that - it wasnt a warm bottle but that was only once in awhile and he was fine because it wasnt cold. But the 1/2 hot, 1/2 cold works like a charm. Once we discovered it, we couldnt believe we were boiling a bottle for that middle of the night feeding, it was so much faster to just boil up 3 oz of water (when he was drinking 6 oz bottles)

I hope this made sense and helped. If you're using ready to feed formula, Im not sure, we always used the powder.

I also have that car warmer thing and I have to tell you that it STINKS!!!!!!!!!! The first time we used it was when we went to BRU to take Gianna's pix and she had a meltdown, so my DH went out to the car to heat up the bottle. OMG, he was gone for what seemed forever and she was screaming so loud, the whole store was looking at me. He finally comes in all pissed off because the bottle was still cold!!! I finally made him to to Starbucks and get me a cup of hot water!!!

From that moment on, what I've done is either heat up her bottle before I leave and give it to her in the car (but that works if someone is w/me) or I heat up water and put them in the bottles and use the powdered formula.

I'm not a big fan of the powder, it makes way too many bubbles and foam, but it works in a pinch!

I was wondering how the car warmer was. I'm glad you saved me the trouble of buying one.
I have the warmer that Rainmama posted. It works OK. I was having this discussion with another Mom in the mall & I was curious to see what other Moms did. I agree with the powder. I tried it once for a few days & it was the most miserable few days for both of us. It doesn't agree with my baby.