Probert seems to be working with the people of Federation Models, who also worked with him on a kit of his original TNG shuttlecraft. (Scott Spicer mastered the shuttle kit, and Jeffrey Waclawski mastered the decals.)
Probert Designs' listing of the shuttle kit
Federation Models' web site
Incidentally, at Probert's Ambassador
's theoretical length of 1721 feet
, a 1:1400-scale model will measure some 14.75" in length! Very cool!