These inside corner sections are designed to make building 90 degree inside corners on structures easy and accurate. They are designed to work with the .040" scribed styrene siding often used on structures. Simply butt the siding up to the inside corner.
They extend behind the siding, to give a gluing surface .080" wide. This will give great rigidity and a very strong bond. I recommend using ACC cement to bond the FUD and styrene pieces, as solvent cement will not affect FUD.