How large should be my port

Hi my friends! I am complete new into this hobby of building speaker boxes and after making a lot of research I still couldnt discover how to solve this question.

I have 2 speakers (125w RMS - Taramps 7driver HD 250S) which have a really really really nice sound (from low-high). I built a box with the following inner dimensions: 30 cm wide, 20 cm tall and 15 cm depth = 9 liters. The wood is 1cm thick however the front panel is not thick so I could easily cut the holes (it's 3mm and I will surely improve the thickness of the front wood in the coming days). Picture below:

I know I should use a much larger box with these speakers, however I dont have any space left for the place where this box will be placed in my house.

My question is this: I already made a hole in the box with a diameter or 6cm to put the port. My question is: how long must be the tube that I will insert into this box in order to get a good low bass (30 hz? maybe 40?). You can see in the picture below I 3D printed a tube with 6cm diameter and 10cm long, but I still dont like the bass (it does not improve much when compared to have the box sealed).