How would you seal old holes in your back rest/storage box?

Hey Guys,

Im looking for some suggestions on sealing up some holes in my rear storage box. I want to seal them well enough that water doesnt leak in while spraying water on it with a hose when bathing her. 🙂
Whats making this more complicated than just using snug fitting stainless bolts and washers, is the holes aren’t actually round.
If you follow this link to my boxes manufacturer, you can see its ment to be attached to the quad with 1 inch wide nylon straps.

Quadraxatv.com, HI-VOLUME STORAGE BOX FOR SINGLE SEAT ATV’S

My box is bolted directly to my rear rack, im not using the nylon straps. Which leaves me with 8 holes in the box, approximately 1/4′ wide and 1′ long.

How would you fill these? Im thinking some tape on the outside of the holes as a temporary backing, and filling the holes level with a black marine like sealant like seakaflex, and removing the tape once it cures. But im not sure if it will stick well to the plastic box (I have no idea what kind of plastic is it?) or if these would be easily pushed through at some point with random things bouncing around in the back.

What would you do?

Any thoughts or advice would be greatly appreciated!
Thanks for your time

Alan