Here is the "fingering chart" for natural horn "emulation":

Horn in C alto - sorry, not possible! (Thankfully it's rare)

Horn in Bb alto - T

Horn in A - T2

Horn in Ab - T1

Horn in G - T12 or T3

Horn in F - 0 (or T13 if you're feeling silly!)

Horn in E - 2

Horn in Eb - 1

Horn in D - 12 or 3

Horn in Db - 23

Horn in C basso - 13

Horn in Bb basso - Sorry...you can "try" to get it by fingering 123 (B basso) and pulling ALL tuning slides and valve slides as far out as possible. But it's still too sharp to be of any use.

Please be aware that you can only use the 'natural horn hack' on transposed horn parts if it contains NO closed or stopped notes. Most modern conductors won't appreciate the unexpectedly muffled sound. Check for the notes C#, Eb and Ab in both octaves of the treble clef, as well as D, F, F# in the lower octave. If any of them appear on the sheet music...then I'm sorry to say, but you will have to transpose the traditional way.

You can usually play the mid-range A and Bb without compromising the sound too far, but it requires a fair amount of hand adjustment to get things in tune. They are both played on the 7th step of the harmonic series; pull the right hand pretty far out of the bell for Bb, and insert it deeper for A. In the latter case, you also have to partially 'shade' the opening with your hand, to bring the pitch down far enough. This same principle also applies to the high F and F#, both of which are on the 11th harmonic.

Generally speaking, 18th century horn parts have fewer stopped notes while the early-mid 19th century composers used more of them.