I mostly use OPUS nowadays. Just download a podcast/audiobook/whatever, convert it to 32 kbps (which even is redundant! even lower is totally Okay, 24 kbps still can sound subjectively lossless for human speech, 16 kbps will probably be still good yet audibly different) OPUS with the voip profile and fit tons of content to listen to offline on whatever a humble storage your portable device has left.
OPUS really saves the day as today smartphones storage usually is filled with high-quality pictures and videos you have taken and apps getting gigger and bigger every year.
I wish Apple would introduce first-class OPUS support in the M4B container in iTunes but as long as it doesn't (and I doubt it ever will) 3-rd party audiobook players do a great job.