[kenji226]

For the holes where the screws penetrate the plastic, they make washers that are metal on one side, and a rubber gasket on the other. I have used these to mount handles to my carpet extractors with bolts and a nylon lock nut without any leaks.

 

[kenji226]

Here they are at Lowes. Maybe not the right size, but you get the idea.
Shop The Hillman Group 75-Count 5/8" Galvanized/Un-Coated Bonded Sealing Washers at Lowes.com
Make sure you get the galvanized ones. You use them like a bulkhead, gasket on the wet side.