The time domain plots clearly showed that getting some damping around the driver is a good thing. Also some vibrations in the OSB could benefit from damping. So I took 12mm bitumen impregnated softboard and glued 10mm felt to it. The sizes are 3 each of 400x120mm and 400x100mm. The final lenght of the boards were trimmed to size by 20-30 mm if needed.

One 100 wide board was glued to the cieling of the cabinet (I felt vibrations there before) The front wall was covered by one 100 and one 120mm piece ( seen far back in the picture below. One 100 wide pice was glued along the back of the closed end below the brace. Then two 120 mm wide pices was glued to the dividing baffle on the side facing the closed end. Finaly the top of the dividing baffle with the cutout was covered with a layer of felt. All gluing was done with silicone glue.

The nearfield response does not suggest any major changes perhaps a slight flattening of the dips at the harmonics.

Now for the pipe

At the pipe and large changes occur even with this very modest amount of damping material.
The sharp peak at 120 Hz is reduced by about 6 dB
The huge peak at 200 is marginaly affected
The peak at 350 Hz is reduced by 9 dB
The peak at 500 Hz is reduced by 4 dB.

I then put a damping layer in the front section by gluing a 60x40cm soft board below the front braces.  This time with ordinary wood glue as I run out of silicone glue.

If I stagger the pipe output with the orginal unlined in green, the lowest trace. Next up the red is with the lining around the driver. With the frontplate is at the top note that the brown trace, note that the trace is smoother than the other two.

If I alight the bass part of the traces on the big improvement was the felt and board lining. The added front board is icing on the cake and the knuckle rap test is much improved. The dip around 150 Hz is a bit flattend out by the front brace

So to conclude I recomend the design. Building it from scratch I would line all parts in the closed en by felt before putting it all together. The front damping is also a good thing to have.

I did some experiments with wool in the closed end but it is really tricky to get anything down there with the braces and the added panels and felt.

That 200 Hz peak is still there. The current opening is about 160cm2 I plan to block it and open a hole at the very back below the closed end. Hopefully this added 90 degree bend will reduce that 200 Hz peak... or not.

After that, it is time for an open baffle I think.