Don't use epoxy for backing it is not flexible enough, use Titebond, Urac, Unibond or Smooth on.
If you are going to back it, back it now, before you tiller. Get your limbs bending a little and glue your backing on in a reflex.
Lay out your basic profile and go to your finished dimensions after you get your backing on.
Backings have a tendency to move around a little while you are gluing them on, better to have a little wiggle room on your profile that you can correct after the backing is on.