Jim,
I have used a product called acraglass (available from
www.brownells.com) it's marketed primarily for gunsmithing (gunstocks),
but I have had good luck with it in lot's of situations. There is one
form of it (acraglass gel) that is almost like a putty - it should work
well for filling voids. Brownells also sells powder dye's and some
other 'additives', like powdered metals.

I think you will find a number of epoxy based products that will work
for you - watch the price and charcteristics like thickness and
clarity.

Paul