Plexi is going to warp, heat or no heat. It just doesn't have the rigidity to maintain a planar surface on it's own. You can cut some supporting strips, maybe, and glue them to the plexi to try and stiffen it, but inevitably, it's still going to end up bending a little.
Shakey's right - get a glass canopy. Most pet stores sell them for that 10g for about $10-$15.
- Sam P -
plantedtanker in limbo - all tanks currently in storage