Ok, here is a combo idea:

 

Stuff a Depends in a gallon sized (3.875 L)  heavy duty freezer Zip-loc and Marks-a-lot the bag with "Dependably Disposable Pot, Contents not Potable, Expiration Date: ASAP, Do not put this bag on your head as suffocation will be the least of your worries, May be used to assist in Oil Filter removal, Single use only, Keep away from infants and the senile.  Skull and Crossbones art is optional.

 

Dom, your device is admirable, if sea-worthy.  Only go inverted if you can maintain 1 G relative to its bottom.  Perhaps you can use expandable spray foam to seal after use?

 

Grayhawk

 

PS Experience = Made just about every mistake, got a few left to go though.