Bass reflex divider missing in DayDream Signature 270 plans – is it actually necessary?

[Audiomeisterei]

I’m currently building the DayDream Signature 270 and noticed something odd: both the plan and the photo show a bass reflex divider, but it’s not listed in the bill of materials.
That’s a bit surprising, since the plans specifically mention all parts smaller than 10 cm — because most (German) hardware stores won’t cut pieces that small.
The only place the divider is mentioned is in this sentence:

“The midrange enclosure including the braces, the top with reinforcement strips, the third brace located at the height of the woofer, and the bass reflex divider are then glued to the back panel.”

So my question is: is the divider actually needed for stabilization and just missing from the parts list, or can it be left out entirely?
And if it is required, would it be okay to make it from 20 mm plywood instead of MDF?

Material	Quantity	Part Description	Dimensions (mm)
MDF 19 mm	2	Side Panels	700 × 325
	1	Back Panel	646 × 362
	1	Bottom	325 × 362
	1	Top	306 × 362
	3	Braces	306 × 362
	1	Bass Reflex Panel	271 × 362
	1	Midrange Enclosure Back Panel	248 × 176
	1	Midrange Enclosure Side Panel	100 × 176
	2	Top Braces	306 × 40
MDF 25 mm	1	Front	700 × 400