I've done many round ones - with screw in rings. I made my own tool to release the glass from a bit of steel bar.
Never done a rectangular one, but have you tried a hot knife to release the bedding compound?
"Travel is fatal to prejudice, bigotry and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome and charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime" Mark Twain... so... Carpe the living sh!t out of the Diem
I'd rather look back at my life and say "I can't believe I did that" instead of being there saying "I wish I'd done that"