Clean Cutting exterior horizontal lines

I have an issue I hope someone has an idea on.
I need to cut straight horizontal lines on existing exterior siding at the bottom of a wall in order to slip a wall cap over a stone watercourse.
Lines have to be about perfect, so I am not doing this free hand.
I can think of several ways to do it, but I’m hoping there is an off the shelf product similar to the fess tool guides that could help me.
There are some long cuts, 20’+
Replies
Make up an 8’ shoot board, a precursor to the Festool and other cutting guides. It will be an opposite shoot board, with the guide strip at the bottom. Drill holes in it so it can be screwed to the siding. Might want to add strips on the back side to lay against the siding bevels so it lays flat.
You’ll have to move it but you should be able to register it to a siding coarse with those strips.
Thanks much, looking into it
Come back and show how your system worked out. Might help someone down the road.
Thanks