from G10 tubing. It is available from McMaster in suitable sizes for screws from 8-32 up to 3/8-16 at least. Pick a size with an ID a bit smaller than the tap drill size. Cut off a length just short of the thickness of the core and deck skin. Drill and tap it for the screw size needed. Drill a hole in the outer skin just slightly bigger than the OD, then decore a good annulus all the way down to the inner skin. Fill the hole with West Six10 or thickened epoxy, plug the end of your insert to keep the epoxy out, insert the insert flush (you can sand it flush if it comes out a little too high).

Here are the inserts prepared. In this case I am mounting a bit of hardware, so I have also cut a G10 plate to reinforce the skin laminate and stand the hardware off a little. I sanded off the paint and drilled the holes to the bottom of the core. This is two 1/4-20 and two 5/16-18:

Because the location is critical and fixed for hardware, I carefully glue the inserts to the plate with the hardware in place. This guarantees alignment and location:

Ready to install. I have screwed bolts all the way to the bottom of the insert, backed out about 2 turns, and filled the end with hot melt glue. This is the easiest way I have found to plug the ends so that epoxy does not fill them. Also note the circled pip in the outer skin hole. I want a hole in the skin not much bigger than the OD, but that will not let the excess epoxy exit, so I have made a little exit groove for it with the Dremel in each hole. Also taped off the area so that it can be cleaned and the tape removed immediately before the epoxy gels. Remember to sand the OD of the inserts before installing:

Mounted and ready to clean up and paint. It is a pretty strong installation, with about 2 bolt diameters you will break the bolt first, or perhaps delaminate the whole area. The large annulus of epoxy is tying the two skins together and bonding it to the insert, replacing a backing plate to some extent:

If I don't have the G10 pad, I stand off the piece to be mounted with a plastic spacer which will not stick to the epoxy. You can also just stick them in with a long bolt, being careful to not get epoxy slopped on it. You need to be careful to get them in square, once the epoxy gels they are where they are:

Here is a finished single 5/16-18 created by the above picture:

Since they are completely imbedded in epoxy and do not penetrate the inside skin, there is no possibility of either core or boat leakage. I have done these in various sizes for a lot of things, including winch mounting where the back side was inaccessible. If you strip them somehow, you can redrill and install a new one. With something like a 1/2" OD and 1" core, you have about 1.5 sq in surface area glued, loaded in shear - should be good for a couple of thousand pounds at least. I sometimes use these on the back side of a single skin, if I don't want or have room for a backing plate and nuts.