When you need provisions, the Engineer's Dispenser is key. On the game, the dispenser slowly generates ammo and metal for your teammates and can also heal them, albeit much slower than a Medic. Put a couple of well-placed sentries around it and this could turn into an impenetrable wall. The Dispenser is a part of the Engineer's buildable structure, along with the Sentry gun and teleporter. To download the pattern, click the link below and scroll down to the bottom of the page, there's four PDF files.
Team Fortress 2 Papercraft - Dispenser [Highwaychile]