I am going to make a sub-set of cards that will take the adventurers onto the walkways,but only for a short number of turns. First there will be a Ladder card, which will be shuffled into the Caverns deck, which will then start a sub-deck of maybe around 5 cards at most. The Ladder card will instruct the players to draw 1D3 +1 cards from the Walkways deck, the last of these will always trigger another Ladder down card at the end, back to the Caverns.
So these are the walkways I've made so far, they are influenced by the first Hobbit movie and other models people have created. They are made from MDF and tapers so they are quite strong but reasonably light.