Life is full of compromises. I've faced similar size constraints and modified flat packs accordingly and they worked for their intended purpose.
It sounds like your going the flat pack route and you aren't going to want to change dimensions in multiple directions, or else your not too far from making the flat pack pointless.
My suggestion is simply to replace both 3/4 inch thick end pieces with new 1/2" mdf pieces, while also insetting them an additional 1/4" on each side. The change of material saves you 1/2" and the 1/4" inset on each side saves you another 1/2" total, moving overall length to 19"
Yes, the 1/2" end-caps might have more flex than 3/4" material. They are small pieces, fully captured on all sides. The world is full of speakers built with 1/2" cabinets. That small change won't "ruin" your speaker. The loss of 1/2" length of internal spacing may cause a minimal tuning effect, but may not be enough to even be noticed and I doubt that change would have more impact than its overall placement in the room. Again, Im certain it will not "ruin" your speaker.
Most consumers don't utilize ideal room placement and they still love their speakers. You will still love your X-CS even if you shorten and modify them a tiny bit. They won't be ruined.
I dont see how that small internal volume change would be considered detrimental. It is not necessarily more of a volume change than upgrading a small retail speaker by adding a new much larger crossover board with large inductors and capacitors.
I do think the X-CS is a great speaker and will work well for your situation, even if you shorten it a tiny bit. If you aren't set up to cut new 1/2" end pieces, and can't get any help locally, I could probably cut them and send them to you for almost nothing. I'm on the table saw all the time, something small like that is super simple.
Good luck, whatever you decide
Elon
ezeescrossovers.com
