We have been very happy for the last 10-years with a stitch-and-glue holding tank of 1/2-inch plywood. Coated inside and out with two coats of epoxy, it now looks the same as when first constructed. It even has a six imch see-thru port in the top for inspection and maintenance shoud maintenance be needed. There are no odors and no leaks. They can easily be built to custom fit the alloted space and while ours is only thirteen gallons the estimated cost was about $20 including the bronze fittings. I would foresee no problem with doubling or trippling the capacity. Just add baffels to break up the free-surface area.
This is very interesting!! We have a Pearson 28, and the bladder used for our holding tank has been relieved of duty. We are now looking for different solution. The space that the bladder was in was not an even rectalinear cube. The recess in the lazarette designed to hold the bladder can be described as a cube-and-pyramid-combined space. We haven't done the math yet, but we think the bladder held about 6 gallons.
Question: Does this plywood-and-epoxy solution really work?! If so, this may be a good option since the standard holding tanks which are cubular will take up precious space in the lazarette (since it won't fit nicely into the space previously dedicated to the holding tank).
Sharing your experiences regarding this is appreciated.