Re-reading my reply I realize I should be more definite in my warning:
Li-Poly batteries can catch fire and explode if charged incorrectly. In particular they cannot tolerate over-charge or trickle-charging.
Personally I don't think it's worth the risk to make your own charger unless you take precautions to mitigate the danger (charge the battery in a ceramic container, preferably oudoors?). Lithium fires are very fast, very hot and cannot necessarily be extinquished with water.