I saved them locally for reference, so I'll repost his original post with his pics in place hosted here:
I have had a few people ask me if I would share how I was able to make a nice dome shape out of foam. I am using the dome shape for the top of a helmet and also maybe using the domes for shoulder pads/bells.
Sorry for the amount of pictures but I like to show every step. It is an easy but (I think) very successful way to make them, enjoy.
This is what you will need... two plastic hard hats, I am using toy ones I picked up at a yard sale for 25c each. You will not that on one of them I have removed the little black clips that held the liner in, this is because they would of made marks on the foam. You will also need some tape.
Step 1 has two parts to it, first and most important is make sure the wife is out before using the oven!!!!,, then heat up the foam.
Step 2, place foam over one of the helmets. then place the other helmet on top and push them together, sandwiching the foam between the two.
Step 3, tape the helmets together as tight as you can, you may need some help here. You can see my little helper in the backgroun, he was able to pick up the tape after I had dropped it.
Step 4, wait for foam to cool, or put it in a cold place.
Step 5, Once the foam has cooled take the tape off.
Step 6, remove the two helmets, and trim the remaining foam off.
Here is a dome getting ready to be used as the top of my batman cowl.

Here it is on the batman cowl.

Here is a dome I made for the top of an IronMan helmet.
Hope this helps someone.
Note: I ignored the step with the tape. I just held them together firmly in place, and have a small fan running during the molding. I just let the fan blow the cool air into the helmets, and the foam cools off within a minute or two. Also he recommended using 375 for a minute.. for the thinner rolled foam, reduce this to 300-340 for a minute.. and keep an eye on the foam.. it'll flatten, and then start to bow up as it heats.. when it starts to bow up, it has become moldable.
Run long beads of hotglue on the inside of the domed part to help them retain their shape in the future, and repeat that at a 90 degree angle so you have a checkerboard or fence kind of look to the hot glue beads. when the foam heats up, it can try to go back to its original shape. If you've skinned the foam with plastidip pva glue, or used the inside hot glue trick, it'll be more prone to retain its new molded form over time.