I built one of these last year which turned out really great. The original horn and flare were not available so I ended up using RCF parts. I used a B&C crossover I had to hand which seems to work fine. I don't have any way to measure response so I'm not suggesting it's going to be properly FRFR but I've had plenty of favourable comments from others about how it sounds. I came across the Convair crossovers that Blue Aran sell which might be a decent starting place and are affordable.