I was set on a natural-shaped nipple, and these worked really well. We tried to avoid plastic, so we used the nipples and lids on Philips Avent glass bottles which worked great. If plastic doesn’t bother you, these bottles are awesome.
Lansinoh Anti-Colic Baby Bottles for Breastfeeding Babies, Formula Feeding, Breast Milk Storage - Slow Flow Nipples (Size 2S) - 5oz/3pk
These fit so well in any bag and are really versatile. If you have more than one, you can stack them as high as your bag will let you. We use the top for formula and the bottom for snacks.
Babymoov Babydose Formula Dispenser - Green
After trying so many bottles, this was one of only two my daughter would actually take. If you’re struggling with bottle refusal, these are the best if you are not a stickler for nipple shape.