A 31-week lead time on Nichion MUSE 450uF non-polar capacitor (needed for speaker crossover) is unacceptable.
This long lead time at Mouser, DK, et. al.

I assume creating on non-polar 450uF from two in series 900uF polars is an option.

So:

---------- (pos) 900 uf (neg) -------------- (neg) 900 uf (pos) ----------

Equals:

---------- (NP) 450 uf (NP) -------------

Correct?

 

Sorry I never really tried this. I found a video that may help in this regard.

 

Video still shows a polarized capacitor, with higher voltage rating and lower capacitance (1/2 of equal value caps).
Original poster's diagram is correct, joining same polarity in the middle, will give nonpolarized capacitor "equivalent" with 1/2 the value of two identical caps.

 

Correct?

Yes.

This is from Cornell Dubilier:

 

It is not likely to find a 900 mfd capacitor of any flavor. so use two 450 mfd in parallel in each direction as shown in the drawing.
And what sort of crossover requires that much capacitance???

 

In terms of distortion, damping factor and avoidance of clipping, it would be preferable to put the high pass filter on the input to the amplifier, not the output.

 

It is not likely to find a 900 mfd capacitor of any flavor. so use two 450 mfd in parallel in each direction as shown in the drawing.
And what sort of crossover requires that much capacitance???

Given the likely tolerances available, two 1000uF caps in series will do the job.