I recently acquired a Model 1920 arms rack, then rigged for 20 M1911 pistols and 20 M1903 Rifles, which I hoped would hold my Krags. It had already had several different holes (some regulation, some not) drilled in the main column, so, when I discovered I needed one more, it was no problem, and was the only invasive modification. Getting the pistol ring off, with its soldered joint, was.
To get all of the muzzles at the same height, so as to fit the locking ring, I constructed a platform (exactly 8 about the butt ring) from 3/4 plywood and four short lengths of 2x4, which fit the butt recess without modification. The center support for the plywood is a PVC coupling, slightly trimmed.
The full-length rifles rest perfectly, requiring a pull of a few ounces to remove them from the notches. The carbines are a different matter, wanting to fall outward if not restrained. This required the installation of small butt shims to kick the weight forward into the ring.
Visible guns include three M1896 Carbines, a Model 1898 Carbine, the SRS listed Boxer Rebellion gun #70210 (restocked at Benicia), a Model 1899 Carbine, a school gun, and two M1892 rifles (which had to have their rods removed - small fly in the ointment). Out of sight are my M92/96, and several 1896 and 1898 Rifles, as well as a thin-wrist 1895(sic) carbine.
Next is to figure a neat way (looking like it belongs, but without damaging anything - including the rack) to hold the two 92 rods, and ten bayonets.
If anyone wants to do something similar, I can provide measurements, etc.