.. Fitting the helmet

Did you notice something special about the teleporters?

Those things make your head smaller!

Using the teleporters you should get your head small enough.

The red and blue teleporters will take you to places where you can walk back to the teleporters. Use both of them (the order does not matter) and always walk back to the teleporters. From the Hall of Heroes you can crawl to Marek and back again.
After using both red and blue teleporters use the middle yellow one to teleport to the containment facility. With extra small head this time. The helmet will fit now.