I would expect that a plastic tank would not pass a survey, even if you properly secure and vent it...have we ever seen a internal plastic tank in a new boat...not that I know of. Maybe our resident surveyor can comment.
If my suspicion is correct, you are likely to find that you end up spending the money to properly install a new alu trank when the next buyer can't get financing or insurance on your boat...you may have trouble with your insurance.
Gasoline is dangerous, probably not an area for the handyman to have a learning experience.
just my opinion.
Certified...in several regards...