Those weren't my suggestions, just a few things I've seen put forward by others as worthy of consideration.
If these men are so unpredictable that they couldn't be housed in female prisons without significant expensive reinforcements, why are they deemed fit to roam the streets? Just my opinion, but if the cost of reinforcement can't be met, any cell is better than free as a bird, until more prisons can be built.
I think we're in 'choose your hard' territory now. Any ideas need to be weighed up against releasing violent men back into the community.