Joining MDF Frame at the corners

[TealaG]

When I moved into this house, I ordered a Mirror mate mirror frame.  They precut to your measurements and cut some "grooves" that accept their special fastener.  When you get it, you're supposed to glue the plastic fastener into the "grooves".  They aren't literally round grooves...it's a funny shape I haven't seen before and it matches the fastener.  This shape had "ears" that the plastic fastener fits in.  The "ears" broke off and the fastener wouldn't hold.  I then tried a right angle brace, but there was a funny gap at the 45deg angle.  Ultimately, MirrorMate made another one and told me to keep the first set of moulding and I somehow made that work and bought custom colored caulk to hide the remaining flaws (it was still cheaper to do all of this than to have paid the builder for them to do this)

I didn't know before I bought it that it would be MDF or that the "ears" they cut into the MDF would break off.  I had assumed it was wood, but well...I'm naive and should have known based on the price. 

I'm too cheap to just toss it and the mirror was very large so there's a good amt of moulding, even if I cut off the "fastening groove thing".    So, let's say that I have the proper saws/jigs to get the angles recut so that it'll be "perfect".  What method of fastening would you use at the corners?  Using only white glue didn't work well...  It has a slight rabbet on the back so there may only be 1/2" of thickness to work with because of the curved decorative cuts on the front side.

Suggestions appreciated here!!!

 

[MGB]

I work with mdf ALOT...unfortunately

Glue and clamp is the best way. 1/2" of gluing surface should be fine, but you have to clamp the joint.

You could potentially countersink a screw from the top or bottom of the frame. But you have to drill a pilot to accommodate the screw otherwise it will split.

 

[TealaG]

Thanks...you found the joins that I have!  plastic, ears and all!  I now have corner clamps so I might try it again.  THANKS!