There are a couple of common methods used in aerospace. The usual choice is between a 'pan-down' or an insert.
For a pan-down the core is removed locally and the skins ramped down and reinforced (including the ramp-downs) for the attachment, usually a fitting.
For an insert, 'microballoon blocks' can be used. These are usually room temperature-set epoxy with about 10 or 15% of phenolic or glass microballoons. This mixture is cast to a suitable shape to fit into the panel full depth and big enough to distribute the force applied such that the stresses will not fail the core at the block's periphery. Sometimes higher density core is used local to the block to keep the block size and weight down (extra money of course). Foaming adhesive is used at the block's periphery to make the bond to the core. Sometimes a machined Tufnol-type block can be used instead of making a custom block.
If the load is very low you could just attach to the skin with no reinforcement. A light fitting could be glued on or through-bolts used, perhaps with distance tubes. I've seen little clips bonded on for non-structural attachments such as light electrical wiring. As a rather (very?) 'low-rent' solution blind rivets just to the one skin could be used, but that's a bit shonky. I once saw that used for clips for a nacelle fire-wire, where a bond could not be used, since a fire might destroy it. Fluid ingress is a problem for this sort of thing.
I have seen some very fancy inserts used which were not full depth. This was to absolutely minimise block weight. I didn't think that the very small weight save was worth the complication, but the designers (Dornier) did.
Another alternative I've seen used for moderate loading is 'cotton reels,' basically a small cylindrical block for each through-bolt. Locating these in the panel with sufficient accuracy is usually a problem.
NB: larger blocks can be a problem because of disbonding from the skins. It's partly for this reason that pan-downs are common.
