A couple of comments that I hope will be helpful......
When mounting the fragile lens element it is important to not apply point source pressure to it as this can promote a fracture, especially if the holder changes temperature. To avoid this it is important to have a small clearance gap around the lens element outer edge and to mount it between two soft ‘washers’ if it is not being fixed in place with a mastic compound. The ‘washers’ may be nitrile ‘O’ rings or common fibre plumbing washers. The expansion gap and mounting rings/washers ensure that no point pressure is applied to the lens element. It is possible to use very thin soft washers or ‘O’ rings so they are often not visible when installed. The cross sectional profile of a Plano Convex or Meniscus lens element is not well suited to being clamped ‘bareback’ between metal surfaces.
When making a clamping screw that will impact upon a soft or easily scratched surface, like a lens barrel, it can be a nice idea to use a slightly larger finger screw diameter, centre drill it’s tip and make a nylon insert tip (top hat shaped) that provides the required pressure but not the potential scratching to the clamped surface.
Fraser