How can I determine random start location/start points for modular panels of set widths?

Hi!

I am trying to randomly locate the start of a seam for panels with a set range of widths of 6ft, 12ft,18ft. Height for each is 9in. How can I ensure that the widths I am getting are those I listed above? I thought I would do this in the domain but when I dimension the distances between lines I get floating dims.

The end goal is to have a slatted wall with panels of 6ft, 12ft, 18ft randomly placed across the entire extent of the wall.

Thanks in advance!