What keeps your average home experimenter from building an optical clock is the fact that femtosecond combs are still way too expensive and exotic. Some progress has been made -- you can get them from ThorLabs, for instance ( https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=11... ) -- but they are still in the "Call for pricing and lead time" category.

Once optical comb sources are commoditized to the extent that solid-state lasers are now, a lot of fun stuff will become possible.