If you really really want to tile onto plastic (I wouldn't), the first thing you need to consider is wether or not the plastic trays are stiff enough. If they are, you need a really good primer which explicitly state it's suitable for plastic substrates in wet environments. You need a high quality flex adhesive and grout as well. I'd advice against using large tiles, as they're more likely to crack if the substrate moves from weight or some such.
That's pretty much the advice I can give.
Edit: You probably want to abrade the plastic surface, and clean it very thoroughly first as well. Also, rinse the surface VERY thoroughly with lots of clean water and let it dry completely before priming.
