Gantry X axis technology

For x axis lengths of 16 to 20 feet, are there any non-custom machined ball
screws available? I have not seen any. The longest standard I have seen is
12 feet.

I have looked at the Bishop Wisecarver Lo-Pro timing belt based system.
However, with a 20 foot axis length, I have been told that zero backlash is
not possible. It is possible in shorter lengths, like 6 feet.

What technology have other long x axis systems utilized and what compromises
have they lived with?

Thanks,
Joe Fahy