The wood/epoxy tank will work if done right... I will assume that you
can build the tank on the workbench and then install... The key is to
have a stiff tank to keep the epoxy from cracking due to the tank walls
flexing, encapsulate the flanges of the hose connectors, and to have
every millimeter of plywood heavily encapsulated in glass...
The method is to use multiple fiberglass layers lining the tank... A
tank built from half inch plywood with a quarter inch of fiberglass
liner will work, along with adequate bulkheads to stiffen the tank and
control sloshing... Vacuum bagging the fiberglass gives the best
physical properties... Use WEST SYSTEM 105 resin and slow hardner...
Slow hardner gives you the best physical properties in the cured
epoxy... Rapid hardner is less good and 5 minute epoxy is not
considered water proof... Vacuum bag it and apply heat during the
cure... 90 degrees F for the first 4 hours then 130 F for another 8
hours.. Do not overheat during the first 4 hours...
Be sure the fiberglass/epoxy liner wraps the raw edges of the wood at
the access hatch... After the internal tank is cured, then trim the
glass at the hatch opening as necessary and put a layer or two of
fiberglass on the exterior, including lapping down over the hatch edges
so it is doubly covered and will leave the wood totally encapsulated...
While the external epoxy is curing, put the hatch cover in place, wrap
it in plastic so it doesn't stick, and weight it down so that the fiber
glass forms to the mating surface of the hatch... Use a neoprene
gasket for sealing the hatch...

All hose barb connections have to be installed at the time of glassing
and the flanges buried in the glass internally and externally,
including saturating the raw edges of the holes for the hose
connectors...

Done correctly the tank will be servicable for the life of the boat...

denny