What if you just move the middle wall to the centre of the port? I'm no box pro but if this is your first attempt at a ported box and you're not trying to do a tline box, why not check to see what happens if you keep the port equal on both sides?

 

and reduce the length of the inner port board to match the port width.

 

 

Without actually doing the math (I hate using cm...), looks better than the first design.

 

What's the tuning you're looking for?

 

So... 69 cm deep... 50 + 18 = 68 cm... 69 deep - 68 (port board plus port width) leaves 1cm.

1 cm = .3937" Thats not 3/4 MDF.

Like Jalat said, let someone design it for ya.

 

Well im open for design plans I could use one for my BTL too

 

try this http://www.reaudio.com/speaker_box/LPort_Box_Calc.html

 

or just use basic math ?gross internal volume L*W*H/1728= cubic feet.... Subtract sub and port displacement = net sub volume. That sub sounds really nice in sealed box but id also put in ported box tuned 25 hz w a port thats 30 -40 sq in of port
IF slot port, can also slide more wood in to shrink port volume but cant make it bigger if needed.

 

I'm for basic math, I'm oldschool!