What considerations apply to material selection for cartridge mounting bolts?

I have found myself needing some longer bolts to relocate a Shure V15 Type 3 cartridge to a Holbo air-bearing system.

The Holbo tone-arm is a tangential tracker with a rigid rectangular 'launch pad' for the cartridge.  The pad is 3-mm thick which is much more than the fixed SME head-shell my dad bolted the Shure to some 45 years ago.  If it was supplied with longer bolts, they disappeared decades ago!

I will most likely have a similar issue with my Audio Technica VM540ML cartridge which is probably a better fit for the Holbo.  It was supplied with a head-shell 4-mm thick, but the bolts slots are recessed by over 2-mm.

A quick internet search turned up bolts made of stainless steel, titanium, aluminium, brass, plastic and nylon.  Some brass bolts are gold-plated (for corrosion resistance presumably).  As a one-time metallurgist, I know that stainless steels can be non-magnetic, or magnetic.

Plastic and nylon are lightweight insulators and immune to electro-magnetic effects like induced eddy currents.

The lower the material density, the lower effective mass of the cartridge.  Here brass is clearly the worst, being denser than steel and weaker than the other metals.

I presume that the main engineering requirement is to firmly couple the cartridge to the tone-arm but I have no idea how firmly.

The Funk Firm has an opposite view with its Houdini coupler which in effect splits the bolts in half, with an elastic suspension between the cartridge body and the tone-arm.  It seems to allow the cartridge body to swing easily to the left or right side.  Does anybody here use these?