What I'm doing is:
- no long rests in jungle hexes (or badland or swamp hexes for that matter either). Rivers are your friend. (Coastal hexes too, mainly because otherwise the party would just sleep in a ship). This automagically makes the game work like intended - that is, the party will MUCH more often have to face those 6-8 random jungle encounters between long rests that the DMG expect. Explain this as part of the magical curse rather than mundane environmental factors - you don't want them to circumvent the challenge, you want them to overcome the challenge!
- all those niggling environmental rolls and checks reduced to simply "if you have heavy armor, you have at least one level of exhaustion, end of discussion". Don't encourage the meta-game of "how will we use skills and magic to dismiss the penalties so we can adventure just like back at the Sword Coast". Instead, make the players embrace the fact the jungle is different - just like the previous point, its penalties are something to be endured rather than dodged, or why even have them in the first place!?
- don't start the clock just yet - if players know they're on a timer, every encounter will be judged by a "can we skip it" mentality. And that's just anathema to a good old-fashioned hex-crawl, where the joy is in the exploration itself! So hold off Syndra and her quest. Ideally a player character will die half-way across the continent, and then you have yourself a much more personal and engaging timer!
- this means everybody gets to die.. once! You WANT raise dead to work on the player characters, since this means the wasting disease gets up close and personal. A better adventure hook can't be found! Saying "you need to roll up a new character" is a HUGE WASTE in my opinion.
