I second lego its the best square edge you can get and pulls off PVA nicely, put 4 ( one in each of the 4 corners and its dead on.) I then use cheap clamps to hold. small metal engieer squares are good, but very expensive. look for LEPIN on Ebay
As for glue, I go either straight out of the bottle or brush it on 50/50 with a hobby brush. As its never seen and you want a good amount of hold, squeeze from the bottle on the hole and part and slip into place.
PS dont do all the frames as once,. I only do 2 at a time and wait for glue to dry before continueing.A lot easier to spot and rectify any errors.