Conventional speakers (bookshelf & floor-standers) are voiced to be placed in a room, not against a wall or in a cabinet. They will always sound better away from walls, ceilings, or any type of enclosures.
However, in the environment you're putting them in, you might want to consider Center Channel Speakers. they will work much better than taking a regular bookshelf speaker and placing it on its side if hit has to fit the space, or give a better look, or go a certain way.
For your space, I would say look at the Paradigm Prestige 45C Center Channel - which incorporates a three way crossover utilizing a Tweeter, Midrange, and Woofer combination.
https://www.paradigm.com/en/center/prestige-45c
For a less expensive option, there is the Paradigm Premier 500C Center Channel Speaker that is also a three-way design.
https://www.paradigm.com/en/center/premier-500c
However, in the environment you're putting them in, you might want to consider Center Channel Speakers. they will work much better than taking a regular bookshelf speaker and placing it on its side if hit has to fit the space, or give a better look, or go a certain way.
For your space, I would say look at the Paradigm Prestige 45C Center Channel - which incorporates a three way crossover utilizing a Tweeter, Midrange, and Woofer combination.
https://www.paradigm.com/en/center/prestige-45c
For a less expensive option, there is the Paradigm Premier 500C Center Channel Speaker that is also a three-way design.
https://www.paradigm.com/en/center/premier-500c