Bill's correct about the spacing; you can't have wide tweeter frames. I used a dozen 3/8" domes per side that were frameless round jobbies that I glued into holes in the baffle, so the dome to dome spacing came in at about an inch, which is good to about 10kHz for wavefront integration. Above that they comb, but only on the vertical plane of course, so that's a minor concern. BTW, they cost me $0.27 each.

These speakers cost me less than $60 for the pair, lumber included, and they sound quite literally like a thousand bucks.

Go with no less than 18dB slopes. Assuming the midwoofer and tweets are in lines that are side by side keep that distance as tight as possible and you need at least an 18dB slope to minimize crosstalk between the elements which would lead to serious combing on the horizontal near the crossover frequency.