Hi Avaiojet
Like your project. Cool idea and I like GBs.
Have you found a 3 line system?
J Roberts made them as Sturdy Built in an upright mount and a suspended mount versions.
The type of handle is not super critical if I remember correctly the local scale fliers(many many moons ago) used various including home builds.
The biggest key is the floating bell crank mechanism the keeps tension on all 3 lines balanced.
Think I have 3 or 4 of these units.

For your lead out guide-Well heck easier to make and scan a drawing ...........................Sloppy Quick drawing
.025-.05 thick aluminum should be enough.
Drawing not to scale! and in fact removable, adjustable line guide is to long in chord length.

David