Will save time and money in the long run.  Especially useful in hot climate.

Features:

• delays the epoxy mix from kicking — saves reagents, gives more time to work
• adds low point of gravity to the mixing pot — minimizes chance of tipping
• creates a brim — protects from epoxy running down onto the working surface
• reduces amount of fumes emitted by the mixture — safer for lungs

Variations:

• 16oz mixing pot by West System (805)

Printing

No supports needed, but a small brim wouldn't hurt.  The less infill %, the better, but make sure you have enough support to prevent horizontal layers from sagging, use thicker walls to ensure there's no leaks.  Slower speed is recommended.  Ideally, it should be printed using minimal layer height, out of something rigid that won't deform in the sun.  The main task is to make sure the print comes out waterproof.  If the result doesn't appear to be waterproof, it may be worth coating it in epoxy before proceeding filling with water.

Could be printed out of flexible filament, injected with water using a syringe.

Filling with water

After the tray is printed, drill a small hole inside the inner wall half-way up, fill it with water, freeze it, and then close up the hole after first use (with the epoxy mixed inside the pot that it was used to cool).  Feel free to later coat in epoxy for better contact with the pot and overall strength.  Be careful not to put too much water inside, as cooling the mixing pot above the epoxy level will form condensate, spoiling the mix.

If you feel adventures and have a CoreXY printer, you could try to heat up water to about 60–80ºC and carefully pour it into the print with a syringe, “trapping” that water inside while it's still printing.  I'm not sure it's a good idea, but might be a neat trick to try.