First up I took a close look at the Chamber of the Tree in the Fiery Sanctum where the baddies constructed their big portal opening device. At 1 inch (25mm) to square on the map it would just fit on a 20" by 30" (500mm x 750mm) poster board.
I got some wrapping paper with 1" (25mm) squares printed on the back and taped it over the board. Then I used that grid to help free hand draw the main outlines of the cavern walls and lava channels. When that was done I used a craft knife to cut through the paper along the traced lines to mark the actual board. Removing the now shredded paper I went over the cut lines in the board with black felt tip marker to outline everything. Next up came yellow fill with red and orange swirls for the lava channels.
For the lava rock floor I took the opportunity to use up some old gray acrylic paint I never really liked the shade of anyway.
For the rock walls, I started with some cross hatching...
...then decided it looked totally awful...
...and painted over it with some brown so nobody would ever notice.
At the game I added four generic markers to portray the supports of the huge tree-shaped device. I'd made these a while back by gluing unpainted wooded pegs onto small square wood bases and painting them gray. They're handy for showing columns, trees, weird tall creatures, etc.
...and then the huge demons charge out of the shadows and attack!!!!
Our next game session will see if our heroes can fight their way back into the cavern in time to stop the activation ritual now underway (which they won't even notice is happening until a round or two from where we stopped last session).