I have helped several troopers by making a second layer reinforcing for their shoulder bridges, it quite simple and has proved to be very effective as a repair or making the addition before it cracks or breaks.
In this instance it was RS armour that had cracked almost through.
I used 1mm thick HIPS on this set, first I worked out the right amount of curvature, then used a hot water bath to create that curve.
Then i marked in pencil the amount of room I had to create the overlap behind the chest plate. This makes sure the weak point is well and truly strengthened.
You can use a heat iron or even the edge of a standard domestic iron to heat along that line and create the step so it all fits seamlessly together.
I create a slight bend then quickly press it in behind the chest plate and bridge to get it just right.
The piece of HIPS i make sure is slightly wider and longer than is required. (only a few millimeters to help with minor alignment adjustments.
I then use CA glue and a kicker to glue it on, Set the piece in place with the CA glue, use a jell type so it doesn't run onto your armour or simply cover the surrounding area in painters tape to protect from any potential runs.
Clam in place and hit with Zip kicker for an instant bond.
Once set, all you need do is sand down any of the overhanging HIPS till flush with the bridge and you are done.
Almost invisible at close inspection.
Hope this helps.
In this case I did both to make sure the other side didn't crack.