2 paires of right angle magnetic clamps and 2 pairs of magnetic clamps with adjustable angles.
Simply pull apart, place inside and outside your corners and snap into place.
The snap and glue pairs hold the two pieces of plastic, wood, etc. in place and allow you to glue them at the desired angle without further worry.
After curing of the adhesive do remove easily the Snap & Glue magnetic clamps.
Clever
One of the most useful tools I've every purchased.