In response to several requests for a cheaper version of the MBOW1 second order kit, I have optimized the high pass section for the Morel MDT20 tweeter, which Madisound sells for $50 a pair.  That brings down the approximate cost by $100 - $170 a pair for the MB20 vs. $270 for the MBOW1.  The extra $100 gets you a somewhat more spacious sound.  But to my ears the MB20 is the best sounding budget design I have done, thanks to the second order acoustic slopes and the solid quality of the MDT20.

The cabinet and driver cutout details can be found on the page for the MBOW1.  The face plate of the MDT20 is the same size as the TB1 tweeter, so you will not have to change any of those dimensions.  You'll see what I mean when you get there.

The low pass section is identical to the MBOW1:

Note that those little resistors (with a value of less than .5 Ohms) in front of the two woofer inductors don't really exist.  The values simply indicate the DC resistance of the inductors themselves (but even those are approximate).

The high pass filter uses the same topology as the MBOW1, but with tweaked values for a few of the components:

And here are the measurements.  The on-axis response is quite smooth except for a dip in the MDT20's response around 6.5k.  The design axis is the top of the woofer frame:

Here is the response 15 degrees off axis horizontally:

Here is the response 30 degrees off axis.  Not as good as the OW1, but I just saved you 100 bucks:

And here is the reverse null.  It is taken with the tweeter connected with positive polarity, which would throw the response of the tweeter 180 degrees out of phase with the woofer around the crossover region in a true 2nd order acoustic design.  As can be seen, the target slopes were achieved over a very broad range.