over time as they evolved back in Europe they added a belt hook. The pistol was stuffed in the belt the hook on the outside, prevented it from slipping through.
These pistols as were percussions were carried uncocked. so were quite safe to carry loaded.
Many of the Kings "calvary" , known by different names depending on the country, did carry two in a pommel holster, along with their sword / cutlass.
But their saddles were somewhat different than the western style we have in America today.
As time evolved again and we came to the day of the percussion revolver. Many early carriers did the same, carried in a belt. or even a soft "buckskin" type pouch that resembled a holster.
And then in a more traditional holster.
And the smaller pistol in coat pocket.
When Col Walker designed the Colt Walker, it was meant as a pommel mount revolver. And so was the later full size dragoon.
Remember the average stature of a man back then was closer to 5'6" - 5'8".
A few did carry on the belt, but they were meant for battle on horse back.
It would be a relatively simple matter to make a simple one. diameter of the saddle horn top for a slip over, or the shank for a snap around.
how far from the horn to the pistol grip.
overall length of the pistol, length from front of trigger guard to muzzle. thickness of barrel including the stock top to bottom and sideways.
Is the trigger to be exposed or concealed.