Upright, unloaded, flattened and strapped to one of the rear panniers is the only feasible and safe way I see to do this.

Hopefully it has a removable belt....

Now for a real world common sense question- What on Earth do you need an 80L bag for? I understand you might go on hikes, but I seriously doubt you are going to go for a week or more after being on the bike that would necessitate an 80L. A 30-40 at most would be better.