Judith in France wrote:
judithsmwrote:

And absolutely key - make sure that all joins are "square" when they
are supposed to be.
Even go to the lengths of working out the internal hypotenuse
(Pythagoras) and then cut a string to that length to make sure both
diags lengths are exactly same. This may seem over the top - but I
think it makes all the difference!!
Thanks Judith, all advice gratefully received.

Another vote here for make sure all right angles are 90°. Didn't at
first "89-91 will do", couldn't get the glass in
properly (didn't fancy bashing the glass in till it fitted) so rebuilt
with 90° right angles and everything fitted