Andy - Unaskedfor advice, given in good faith: I would find it nearly impossible and at least thoroughly frustrating to design a first-order, phase coherent loudspeaker. It took us decades to develop and improve drivers to eventually cover 7 octaves with good enough behavior so that the long tails on the crosspoints would blend properly without over-excursing, resonating, burning out and otherwise misbehaving. The early custom drivers were developed with Vifa, who is no longer in business. Then we developed and made in-house, which are likewise unavailable. Since first-order xo is generally considered unfeasible, I don't know that suitable drivers are out there to be bought.
I suggest you consider other options. A sweet alternative is the second-order crossover which manages phase integrity within a broader window, but to my ear does the job quite musically. Lots of high-end companies, as the lovely Thiel 02, take that rout.
Please keep us posted on your progress, whatever you do.