The original holding tank was very small and installed under the seat in the shower. It had many holes in it when we bought the boat and was useless. It also was not possible to get a new one in it's place without destroying the boat.
I cut cardboard pieces until I had each part of a new tank mocked up to fit in the odd shaped area in the bottom of the hanging locker. This was mainly wasted space anyway. Then I went to the welding class I was taking to get all the boat projects done. With an old road sign as building material, I welded up the tank and fitted it into it's cavity.
You might notice that there is a cleanout in the top. If it ever develops holes, I can glass it on the inside and avoid replacing it.
And that is more than I ever wanted to know about holding tanks.