Rookie needs help!!

Hey guys, I have no experience with any type of construction, however I have read alot and purchased all the tools needed, and am well into the project.
If you see anything I have done wrong thus far please tell me!
Im currently getting the shower walls ready for Granite/slate.
The tub was not alighned with the walls so I built out the walls so the 'Denshield' would go over the lip of the bathtub.
Im almost ready to put up the Denshield, however I have run into a little problem with the top of the shower. There is a Metal structure beam there that has a square up top of it (see pictures), its about 5cm lower than the wood beam there.
Im afraid if I put up the Denshield like this, then there will be a little uneveness where the square of the metal beamis on the right side.
I tried to correct this by putting a few 'shims' there, and now the 'step' is gradual now, but fear it will still be visible.
I think the way to correct this is to put a 5cm sheet of plywood covering the whole top to make everything even yes?
However I was trying to avoid loosing 5cm of the roof which is why I was trying to find a way around this, hense the shims. (I hit my head on the roof as it is
Any suggestions?
Also what do you think about everything else in the picture including the 'niches' I made?.....
thanks~!

[URL=http://imageshack.us][IMG]http://img180.imageshack.us/img180/9696/newebayitems069rw5.jpg[/IMG][/URL]
[URL=http://imageshack.us][IMG]http://img102.imageshack.us/img102/9605/newebayitems070tb3.jpg[/IMG][/URL]
[URL=http://imageshack.us][IMG]http://img241.imageshack.us/img241/6537/newebayitems075tx6.jpg[/IMG][/URL]
[URL=http://imageshack.us][IMG]http://img58.imageshack.us/img58/3044/newebayitems078en2.jpg[/IMG][/URL]