From Dave Graffam Models
PDF – $4.95 USD
Above all, these tunnel sections are designed to be versatile and easy on your budget. I've tried to keep them simple enough that you can use them how you want, but with enough visual appeal that you won't need to add much in order to have a great time playing on them. You can print them on any paper you like. I like to print them on plain index-weight cardstock. You might want to experiment with parchment-style paper too.
Cut your tunnel sections and fit them together however you like. Use low-tack repositionable scrapbooking tape to hold the sections together.
For just pennies a page, you can be guilt-free as you draw doorways, stairs, torches, fire pits, trapdoors, debris, and other features directly onto the pages. You can easily create one-use-only passages and rooms with a lot of unique features specific to an adventure. If you want to add color, plain index-weight cardstock is a fine surface for colored pencils, markers, watercolors, and acrylic paints.
Cereal box cardboard (and just about every other kind of grocery packaging cardboard) is a great mounting material for tunnel sections like these. All you need is a glue stick and a good set of hobby scissors to make your tunnels sturdy enough for years of use. If you prefer, you can laminate your tunnel sections and draw on them with dry-erase makers for endless reusability.