Hi, Ecca,
I've had this problem a couple of times with Gibson acoustics.
Taylor solve the problem (they come with a button installed) like this (and it isn't bad - my son has one):
http://www.frets.com/FRETSPages/Musician/GenSetup/StrapButton/StrapButtonViews/heel.JPGThe solution I favour for Gibsons (as used on my J-50 and J-200) is this:
http://www.cottenmusic.com/images/Misc/Anotolie%20SB%20and%20heel.jpg...though the button should be mounted closer to the "back" of the heel and the back of the guitar (if you see what I mean). The strap is kept tight to the body by the weight of the guitar (rather as it is on a Fender) and is more secure for that reason.
Whatever you do (subject to what I say below), don't use the position that Gibson do on the 335 or SG - at least, not with a tiny button like Gibson use. I guess you already know how easy it is for the guitar to fall off the strap unless locked to the small Gibson button. However, one possible solution for an ES-335-type position would be to purchase and use a Gretsch strap button, which allows secure fixing (the button screws on and off a central post and allows use of a strap with a very small aperture for that central pin).
http://www.gretsch-talk.com/forum/gretsch-discussion-forum/12446-locking-gretsch-strap-screws.htmlHope this helps,
Best of luck,
JN