If you really wanted to stick with the project, you could buy the crossover
mentioned in the design and reverse engineer it. Or look into this one and see if you can predict the response.
It has better components and would probably sound better.
Looking at the PE page on the design, the design would likley benefit from a notch filter somewhere around 8 or 9khz to dampen that spike, and something would have to be done about the dip in the 2.5khz range.
My opinion is that if you want a garage speaker, this would fit the bill. If you want to design your own crossover look for better drivers. I'm not sure these would benefit from the extra effort.