The problem with filler is what is used to make the filler. Things like talcum powder are often used, added to the epoxy to give it body. Such stuff may well take up water, so it is not desirable. At the very least, make sure that a sound impervious layer finishes off the job. Gelcoat is good, but I know others have used paint. I have not used paint for this type of job, so I cannot offer any advice on paints. If you use gelcoat, do not apply it layers that are too thick.
Re the fuel tank in the hull; I think it inadviseable, unless you are just going to sit the tank in the hull void, and be able to extract it at will. If permanently sealed in the hull it would need to be firmly fixed down, with no chance of 'fretting'. I am not sure how you would securely fix a plastic tank into the hull. Understand the issues with deck space, have you thought about a temporary or otherwise fixture in the console?
So, was it you Matt that sabotaged the greatest literary efforts this side of 20.00
Thats it, worn out now, not going to post for many a long day, just sit and admire that nice round number