Bert2018

Can you enlighten me a bit? And any suggestions on what to put under?

Bought new house. Own a pair of I think RF7 floor standing speakers. Nowhere to put on floor in den. Only option is to lay horizontal on top of built in bookshelves. Any ideas to make it work? Get foam or rubber to lay on? Try to tilt down? Tweeters in or out? Any help appreciated,,,