Question 2/840 Enclosure built dramas

What have people experiences been with building there enclosures. I am assembling the 840 enclosure and finding the black corner pieces for the doors incredibility difficult to get it. Of the 48 that I have done so far, I have managed to break 3 of them. One of them broke when trying to insert one of the acrylic sheets, One broke as I was using the mallet to insert one of the 413mm frame pieces and one broke when using the mallet to insert in the small bottom pieces.

I built the 420 version and had similar problems. Some of the plexiglass sheets were over thick and were very difficult to fit. Same with the corner blocks despite debugging the Ali profile also broke.