"We" wife and I, planted the last 12 acres of spruce 9 to 18 inches tall with a planter that I have know Idea of manufacture, but looks lot like a red boss tree planter.
I built a measuring wheel attachment to "click" every 8 foot, the spacing of the planted trees. The planter came with a 6 foot clicker, copied that design for the 8 foot one.
After trials, errors and such, we chit canned the clicker wheel, was not consistent with spacing, ended up using the ol' row marker "8 foot" we used for hand planting trees, made rows with that and then the wife would stick a seedling in the planting shoe just before lining it up with the mark from the row marker and last tree planted..... worked o.k.
Like your horn idea, Can't wait to see how it turns out, operates and does on spacing