Like Crystal i use tiles in my tanks. What i also do is to place some papertowels where the *bathroom* area is. This way i can change them when needed without removing all of the decor.
In order to get all of my girls (5 in the top 40 gal and 4 in the bottom 40 gal tank) to poop in one spot, i place their poop where i want them to go and they usually follow suit.
I've got a pic of my bottom 40 gal posted a bit further down the page under the post *My 2nd 40 gal setup*. The bathroom area is to the very left front of the tank.
Red 
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Crested Gecko Zeek:1.0, Ziggy and Zeus
LEOS: Boo: 1.0 normal , Bronx & Nala: 1.1 blizzard,
Lily: 0.1 patternless, Abby: 0.1 albino, Zoe: 0.1 reduced pattern, Dot: 0.1 hypo
New Girls: Flame, Karma, Bubbles and Bell pics coming soon
Chip: 1.0 papillion (small dog)