Here is the tapped fiberglass backing block installed and ready for curing. Let it cure overnight and in the morning go remove the thru-hull, and the flange, then clean them of the wax.
The last step after de-waxing the thru-hull and flange is to bed them with underwater rated marine sealant and install the seacock, hose barb and hose.
You now have a life long backing block, with replaceable studs, and no extra holes in your hull. Simple and easy..
I choose to use replaceable studs because in the event they become buggered you can simply replace them.