Heads up! I made a new, better version of this. You can find it at http://www.thingiverse.com/thing:926015.

I will leave this old one up, but it is no longer supported. I highly recommend you print the new version.

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The plate on a standard bathtub “overflow drain” pulls water from the bottom. This means the maximum water level can never go above the bottom of the overflow plate.
To increase the maximum water level, I created this “High Tide” overflow plate that drains from the top. Now you can add a few inches of water to your bath!
The guide lines on the front help you know when to turn the water off. For example, for me, when the water fills to the third line, I know to stop the water. Then when I displace the water by getting in, the water level rises to just below the drain top.
There are two options: Regular height and “Tall.” Use “Tall” for overflow drains installed especially low.
Use 100% Silicone Bathroom Caulk to get a good seal. See instructions for specifics.

Examine your old overflow plate and decide which High Tide version to print. If the top of your current plate is around 1″ or 26mm below the tub top, use the regular plate. If it’s much lower, use the tall plate.
Print with support structures.
Use the screws that came with your old overflow plate. Temporarily mount and examine fit. This will help determine how much silicone you’ll need.
Remove the plate and fill the back channels with 100% silicone kitchen & bathroom caulk, applying as needed to make a good seal. Let dry overnight.
Right before you mount, dab a little bit of silicone in the screw holes. Mount and enjoy a higher water level!