very cool wooden tank i like it alot... I think you should dark stain to match the grips & seat, then clear coat it. perhaps spar varnish
incase you don't already know this, for not much more than the cost of the box of plugs you can buy a plug cutter, I find the tapered ones to be the best you get a sung fit with no gaps. This way you can cut your own plug to match the species & colour of wood. Another nice trick is to turn the plug so the grain is lined up, this way they really disappear.