A floating basking island for my turtle?

I have upgraded my turtle's home to a 10 gallon plastic box - white, not transparent but I think opaque is the right word. The plastic box is a temporary home, though it is big enough for him right now. He is still less than a 2" res. I am loosing at least 1/2 an inch of water each day because of evaporation so I was thinking of making a floating island for him to do his basking but am not sure how to do this. If I used styrofoam he would probably pick at the styrofoam with his mouth and possibly eat it. I was hoping to find some kind of floating medium and cover it with a thin piece of shale and some rocks arranged to give him a ramp to get up on it. Any ideas? I don't want to fix something to the side of the box because of the evaporation problem. If the basking area is floating, it will always be the right height for him to access.

Thanks! Good ideas...I live in an apartment but I think I have enough room on my balcony for one of the small kiddy pools. I have a smaller basin now that I put rocks in for him to climb on. I have a problem with evaporation. I live in Taiwan and the weather is consistently 90 degrees F and more than 80% humidity. I don't understand why, but when it is humid, water evaporates much faster than when it is simply hot. I am loosing more than 1/2" of water every day, sometimes almost 1". I want to be able to go away for 3 or 4 days, put some feeders into the water to keep Frankie fed and return home and not find a water logged turtle because the water level has dropped so much that he cannot get onto his pile of rocks. Does anyone have any suggestions for making a floating island? I was thinking about a few sections of pvc pipe with capped ends, sealed with silicon...kind of like making a raft...and putting a piece of slate or even wet concrete on top of that, with rocks, etc. I would like to dialog with someone who has done this before to avoid any stupid mistakes. Also, sometime this summer I am going to buy a 70g tank and turn it into Frankie's new home. I want to experiment with the floating island now so that I have an idea of what he needs when I make his new home.