The only place to install a strap button on a Harmony Sovereign is the upper bout.
Never install a strap button anywhere on the neck heel. There just is not sufficient wood to carry the load of the guitar via the strap button. This is to say, installing a strap button on the neck heel usually breaks the neck heel.
You can see below there is a good inch of unsupported cross grain wood in the heel. Harmony Sovereign guitars have the thinnest wood across the heel of any guitar I have encountered. Wish I had a dollar for every broken neck heel caused by improper strap button installation.
Please note: We install strap buttons on request for free on any Sovereign guitar. Free!!