The International Cessna^® 170 Association

To make and sell a part; wouldn't you need a PMA or some other approval under CFR 14 §21.9?
I thought the STC approves the design and implementation but you needed a PMA to make and sell any parts under that STC (XPMods for example).

Now, if someone has an STC they could sell the approved data in order to manufacture the parts and the Owner could produce the parts without PMA.

The 3M VHB experiment is turning out to be just that..an experiment.
The constant pressure of the gas strut WILL, over time, work the adhesive lose or at least allow the bracket to deflect until it interferes with operation.
I took a heat gun to it last night and the defection increased with the piece hot to the touch

The last idea would be to use a gas Damper strut rather than a spring strut. There woudl be no load on the brackets when closed. But you don't really get the benefit of the door being held open for you (it would slowly close). It may not be terrible. Those Dampers appear to cost $30 or so each. Plus socket, brackets and end fittings you're around $80.00 in materials and the inboard bracket needs custom made. That's still cheaper than the STC but you still have the VHB tape to contend with.

That is a more elaborate and more correct explanation. I was being contrary to my usual method and attempting brevity.
I was posting via smart phone.

I only know that because I've been fervently researching Owner Made parts lately.

PMA and STC are often used incorrectly in conversation. As George pointed out David's explanation is more precise. A PMA is the authority to make and sell a part and that part is interchangeable with factory produced parts in those places the factory parts are approved. An STC is the approval to make a modification to an airframe and it might also be the approval to manufacture a part used under the STC but it is not approval to make that part and sell it to others. A STC can also be used to approve the use of a PMA's part in a place it was not used by the factory.

And one other thing of interest. The owner of an STC even though he may be the designer of the STC, can not install his STC on an aircraft unless he is also licensed mechanic and IA to insure the STC is properly installed and proper paperwork executed.

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I had high hopes for this door minder but feared the tape to be the weak link. The damper might actually be the trick because I don't want my door to slam open or slam closed. I also think structural epoxy could be the answer if one could work out how to remove the part without destroying the parts it is holding together.

Of course I haven't given up on the AN bolt I pointed out earlier either.