Thanks to Dawn for this Frugal Baby Tip!
Ok, there's the simple way and the even simpler way. I'll give you both.
Simple. I take an almost empty bottle of whatever baby soap you use with a squirt top. I use Dr. Bronners. Add a few drops of tea tree oil, a few drops of lavender, and fill it up the rest of the way with water. You want very very little soap in there. If it leaves suds on your baby, you need more water. The bottle goes in the diaper bag with dry wipes, and I throw the dirty wipes in the wet bag with my dirty diapers. I keep one bottle at the changing table too.
Even simpler, substitute the soap bottle with a plain old water bottle.