I found out that 1L Dr. Pepper. bottles from around here fit with slight resistance from the plastic trim under the radio. Works great for me!
Seems to be only the bottles here. Heading to Iowa, the bottles ended up being a hybrid Pepsi 1L bottle without the curved lines (way too tall), then in Iowa the bottles were the same shape as here, but were a bit slimmer and taller (maybe 1/4" taller, but it makes the diff of fitting or not!).
Reading the lid, Dr. Pepper here is bottled by a Coke bottler. I'm wildly guessing the ones shaped like 1L Pepsi bottles (with the big opening, even) were bottled by Pepsi.
