I have seen a ratline tool, but reviews say it doesn't work well.
Most shrouds and ratlines have to be tied in place to ensure square and even lines and pressure on the masts.
One suggestion I have seen in two forms, a paper with grid drawn horizontal with scale spacing for ratlines, clip behind shrouds and tie knots till your done. Other version was a stick or piece of cardstock cut to scale height, and moved up each time you finish a row. Both are guides to keep lines level as you go.
On the one kit I have done ratlines, it was simple tie clove hitch, glue and repeat, and repeat and repeat....