It doesn't even have to be concrete hangars, just make unreinforced hangars that serve as heat protection, but in the case of Russia it would serve as aerospace anti-surveillance measures.Building concrete hangars would have been a good thing to do, even metal shed would diminish damage... They had two years to do it.
This would serve as a deception measure against allied space surveillance.
From above it would look like this. Ukraine's allies would have no idea how many planes were at the base or in which hangar.
If the Russians want to take a stronger protective measure, copy the USAF example:
The USAF is developing a transportable hangar capable of housing and protecting aircraft for certain periods during deployments to remote bases and outside the country. According to the USAF, these hangars are capable of protecting aircraft from threats caused by cluster bomb submunitions. Eventually they could also withstand attacks from small drones, like those that wreaked havoc on Russian bombers/fighters.
The estimated cost of each hangar is between 400 and 600 thousand dollars.
The first example case would cost much less than the USAF copied model.