[How to reuse R-15M Bookshelf Speakers]

My couch is against the back wall. I ended up mounting the r15's above the front's and I like that. But I am thinking the rp240's might not be the best rear speakers for my set up.

[How to reuse R-15M Bookshelf Speakers]

Just got myself (2) RP-240S rear speakers for my system and replace the R-15M bookshelf speakers.  I was thinking to put the bookshelf speakers on top of the towers or mount them above the towers up high, but there is nothing to mount the speakers on.  Just wanted to see what would be best for sound.  I have R-26F towers with an RP-250C center speaker.

Thanks