Trigonometry/CAD people/cabinet makers in here, help please!

[MikeHunt79]

Today I've been mostly making sawdust, I'm building a speaker cabinet but I'm totally totally stuck when it comes to working out the dimensions of two of the panels...

I've coloured the offending panels in red, they are the same size...

Top down view:

Top down view, more measurements:

Side view:

Now the thing that gets me is all the angles, I've tried to work it out using a speadsheet but TBH I'm knackard and my brain isn't fully up to gear... So far I know one of the sides is 164mm, that's it!

I some pics of what I've made so far if that would make things more clear...

 

[JBuk]

you want the angle of the piece in red on the top photo? 11.6 deg

 

[MikeHunt79]

you want the angle of the piece in red on the top photo? 11.6 deg

Cheers. What I really need to know is the dimensions of the red panel, so I know how big I need to cut it.

If it was 2d it would be a piece of cake but the 3d-ness is making my brain go funny!