The easiest way and one of the best ways is to, rap your box in one piece and let the excess over hang the ends.
Make your one seam on, one edge. You would most likely want this edge to be on the top back corner so it is less likely to peel.
Then cut away the extra on the ends just leaving a one inch piece to fold in and glue down. You will have to notch the corners like a V shape so that it fits well.
Don't spray glue until you are satisfied with the dry fit.
Once that is done cut a square or what ever shape the ends are and glue it down.
So you should have a square inside of a square edge.
Then brush the seams to make them disapear.
Take your time and dry fit everything before applying glue.
92 Bonneville SSEi - Stock - Remaining repairs needed due to neglect by previous owner - ABS/Trac, AC.
Mods so far: Maganflow High flow Cat, Rebuilt */C
Future Mods: Interior swap for Leath, High Output Altenator, Crazy system, Climate Control swap & Custom Exhaust/Intake.
98 Dodge Stratus - Wifes Daily Driver
92 Dodge Shadow - My Winter Beater