Novavax has also been working on a Beta variant booster (and I believe has moved on to a Delta booster that hasn't started NHP trials yet) -- here's some info from a presentation: https://www.novavax.com/sites/default/files/2021-05/NVAX-WVC... (It's slides 17 and 18)
Pfizer/BioNTech definitely has a Delta booster getting ready for NHP trials, but my Google Fu is lacking today, apparently.