Over 6 years since I asked, I've FINALLY got round to starting my RCF LF18X401 powered Cubo Subs! Unfortunately due to the astronomical cost of birch ply now, I've gone for MDF, but one is nearly finished and am excited to see the results! Have incorporated handles into the bottom bracing which I hope won't affect things too much, but after years of negotiating stairs, I felt this was a necessary compromise! Will post updates when ready (don't hole your breath)
You can adjust the width to 660 mm, without any issues. I would set the high pass at 20 - 30 Hz (experiment a bit with frequency and slope untill it sounds just right), if you're going for maximum power/ output 30 Hz is better.