wing on sells drain hole covers with a one way flap that allows water to flow in but nothing can come out of it. They come in the circle shape for bathtub drains and a square shape for kitchen drains. Our aunt uses them and she says they don't have roaches - she also pours in bleach once a week. We actually have two unused spare ones (one circle and one square) so if you can't find them, PM me. I think they were $69 each.
in addition, before we moved in we:
- hired a contractor to clean out and seal ALL gaps in the kitchen and bathroom this was a very tedious process but well worth the trouble for the peace of mind.
- cleaned all surfaces (walls, inside and outside of cabinets, floors) with vinegar
- sealed the gaps in all the doorframes with foam and/or weather stripping . These can be obtained in the industrial stores along Wan Chai