You should be aware of the effect of boundary conditions as well as rear porting when placing a speaker inside an enclosure. See
this thread or do a search on this forum for "bookshelf" and "boundary".
I have a very similar setup. Due to WAF I must place my speakers into an entertainment center. My rear ported Paradigm Atoms sounded much worse when enclosed.
I'm very happy with my NHT M6 speakers which use an unported sealed design and also have a crossover switch that compensates for boundary conditions. These or the M5 might fit in your space as they run just fine horizontally. Check
www.pricescan.com for some low (probably too low) from shady sounding stores. These may be useful in pricematching to a local dealer or someone reliable like OneCall.
Many dealers will let you take home demos for auditions. This might be a good option to let your ears tell whether any of this might bother you.