The My Bei Bei MT has a pretty narrow and overall small body. I just sold mine so not sure of the dimensions though. It was certainly the narrowest of any I've used.
The only other one I can think of is maybe the Happy Cruiser but it also had a different shape-not just straight- though I think I saw a pic of one she did that was straight??
You might be able to get a wahm to custom size-I've done that often since I like narrow bodies too.
Good luck
Ann
Ann, Full-time mama of 2 crazy kids and a sling'n toddler