The nice folks at some 'Uni invented an optical and electrostatic trap that "locks" Sr ions.
In principle this system would be ideal for something like this.
I looked into DIYing a Rb clock which is possible thanks to advances in SDR and lasers: the LNBs from a few years back will with some modification tune into the Rb hyperfine transition at 6.8 GHz so a bit of RbCl2 off eBay and some argon eg from a new fluorescent tube starter with the SRD made from a modified tunnel diode or used GaAs based Gunn out of a broken door scanner should work.
You could literally DIY an atomic clock this way but it would be a lot of work comparable with a doctoral thesis and making a double oven OCXO using parts from a broken 8088 MB is possible.
Cs is harder but the transition is closer to the range on LNB so you could do the same here however it will never be as good as a proper CSAC or used module with regenerated lamp.