dealing with high humidity

I forwarded your email to my client and he has a few questions:

1. My client will use the ofuro on average once every ten days. I will
highly appreciate any advice from you for care and maintenance. The Philippines
climate is tropical, with humidity ranging from 71%-83%. There will be windows to
provide natural ventilation.

2. Can we use the hybrid type (with fiberglass mold) based on the desired
dimension of the ofuro, and based on the frequency of use?
If yes, please give me a quote using your recommended wood.

With 71~83% humidity, it is enough if you use it once every 10 days and if you use natural ventilation, it should be fine.

Consider that the humidity in the bathroom is not distributed evenly.
High humidity is “sneaky” and tends to “hide” in the corners and under the tub.
It is very efficient to use a fan or air circulator, placed on the floor and directed to the base of the ofuro for 30 minutes after use.
This pushes out the super-humid air without drying the wood.

I would recommend not to use a cover. Leave the wood exposed to the air and monitor frequently, especially at the beginning.
The precautions in preventing the formation of mold are the same even for the hybrid type.
The hybrid type is about 2.5 times more expensive than the solid construction bathtub.
The only advantage in your case would be that the hybrid type does not need metal straps so you can have a cleaner design.