If you place the acrylic corner piece over the larger curved piece (or whichever one would be shape #3) with the seam allowances over lapping, does it not fit?
The templates looke like would fit - starting with the little curved drunkard's path piece, which fit's into #2, which fits into the larger #3 piece.
It seems that the corner piece should only go in one direction, perhaps you have it backwards.
If you've attached the other 3 pieces, you clearly can deal with the curved seams, so that's not it.
Can you post a picture of your block?