Permeability of containers to poop smells
The problem likely is not (so much) the plastic material of your container, either solid, or bag.*)  Assuming there is no contamination on the outer side of the container, most likely the smells are noticed because of closing the container by either knot or lid is not tight enough.
After some search on sibling sites LifehacksSE and OutdoorsSE I learned hikers found multiple solutions to this problem (example).  Assuming you stay on ground and don't climb, a) a poop tube may be suitable for you. In this tube, you carry the waste with you until you find back to a place connected to a waste management / waste water facility.

(credit, source describes both construction and use)
Step #3 of this guide includes an important clue how to reduce the smell.  To quote:

«Before you go out camping, hiking, or climbing, preload the airtight bags with 2-3 small scoops of kitty litter or powder detergent to remove moisture. This will also keep obnoxious odors from leaking out of your poop tube.»

b) The idea to catch liquids and thus smell in a bag filled partially with some of (advanced?) cat litter is commercialized elsewhere, too (example).
Maybe this avenue is working well enough that additional sealing of the lid by an O-rings of suitable diameter and material (viton is a material resistant to many chemicals, but I don't dismantle a vacuum deposition to check this field of application) is not necessary.
*) Although permeabilities of liquids and gases across plastics may be determined experimentally.