As long as the area around the space you are talking about is secure (that is, nothing will vibrate from the SPL) it should be fine.
26" would give it plenty of "workspace". The SLAPS radiator can move up to 4" on explosive peaks.
The ultimate would be if you could find an additional...
My first recomendation would be the SuperNova MKIV 10". It is "MUCH" louder and plays lower than the other two, and it costs less.
Secondly, Pinnacle makes a sub called the "BABY BOOMER" for $895 Which sports two 8" woofs in a 10" cabinet with 600 watts...