This should be very easy.
For the housing, take the original adapter and stick some rolled up paper into the headphone jack, then cover the outer surfaces (except for the contacts side) with silicone gel and let it set. Be sure to have enough silicone from each side, at least a half inch thickness, to make sure that the mold we are making will not deform easily.
I'm pretty sure an off-the-shelf headphone jack can be bough somewhere, but it has to be the stereo+mic kind. I don't yet know where i could borrow wire to create the connector pins from scratch, but after soldering together the whole setup, use the silicone mold to create a new housing with black epoxy.
I know this is somewhat vague, but i hope it gives a general idea. If i had one of those adapters, it would be a little easier.
If anyone cares to make this a large-scale effort, it would make sense to send an adapter to MoldMuse @ Etsy and have her make a silicone mold professionally. (see: Flower Cabochon Mold Roses Asst Sizes Flexible by MoldMuse)
For the best result the adapter may have to be sent disassembled, but then some has to be done anyway...
Then.. perfect and standardize the electronics side, order parts in bulk and make OEM-grade magnetic adapters to sell on Amazon for a reasonable price, ~$7.00.
I'm not sure which side is magnetic, the phone or the adapter, so if a tiny neodymium magnet is needed, that will have to be taken care of as well before we have a fully working prototype...