Rubber molding is probably the least-bullshit way to get it exactly right. Press or roll a bunch of little divots into some reasonably firm material you’ve made damn sure is level, pour in dark latex, get a thin sheet of flexible material with a bunch of bumps. Try to dip that bumps-down into a very shallow pan of metallic paint. Any goofs can be touched-up by hand.
The quick way is to run a paint roller over some bubble wrap.