Thanks man, this was a fun mod I'd like to to see your helmet when you finish it.
Real plastic welding needs a heating element that blows hot air that has precise temperature and can hold that temperature since different plastics melt at different temperatures. That's for welding plastics that need to be structural. So I just used an old soldering iron.
I basically touch the plastic with it between the seam I want to close and move it forward and backward to slightly mix the softened plastic. The seam will lightly be held together after the first pass, make sure not to go through the plastic in this step. Aim to melt about halfway through the plastic. Then get some extra scrap abs, I used pieces I cut off from the helmet itself, and use it to fill out the seam. I just go over the seam with the material and melt it into the seam until it's smooth. Try not to add too much material so as to not make a bump where the seam is on the outside.
The seam will be together after the second pass but still might be prone to breaking off if left in that state. So I go over the back and do the same steps except I add way more material aiming for there to be a bump where the seam is to reinforce it. Let it cool and sand away.
Hope that helps.