Hi, I had the same problem a while back when I was making paper bag goose shells. You know, the "Goose shells for $.05 each!" Yes I did it. They actually are fun to make, cheap and work well but they have one major problem. In order to waterproof them, you have to use spar varnish or some other medium. I actually just used polyester resin which made them as tough as any goose shell I have ever bought BUT they had serious glare. I painted over the polyester resin thinking this would solve the problem and it was better but there was still a lot of glare so my solution that worked very well was to scatter the light off the surface. I did this in two ways.
1) I used the Rustoleum textured spray paints in black and grey.. They are flat, and they have a built in texture that is almost like flocking. Works very well I thought. I almost considered repainting my commercial shells with it.
http://www.rustoleum.com/CBGProduct.asp?pid=31
2) Made my own textured paint. I just used anti-skid additives and mixed it in to the color I needed. It does not work quite as well as the spray on because the texture is not as big. But I bet if you look around you could find bigger anti-skid granules. Heck.. why not just make your own with sawdust or sand??
The result is that the light hits the surface of the decoy and instead of all being bounced back in the same direction, it is scattered. You get really no glare at all. I think you will be impressed.
Flocking is also an option but flocking materials are expensive to do entire shells. I have alrady done a lot of searching for black, white and grey flock. The price makes it prohibitive.
Okay I know someone is thinking.. How do I make a goose shell out of paper bags??
1)Buy cornstarch and spread it on a cookie sheet about 1/4 inch thick. Heat it in an oven at 450 degrees mixing it every 30 minutes untill it becomes a golden brown color. You have now made Dextrine at literally 1/10 the price of buying it.
2)Cut the head of a decoy shell in 1/2 so that it makes a female mold.
3)Mix dextrine with warm water at a ratio where it makes a jelly.
4)Coat the inside of the goose shell and the head with criscoe or other oil. Corn oil also works.
4)Take strips of brown paper bag 12 inches long by 2 inches wide and saturate them in the dextrine. Then line the inside of the shell with the strips. Run strips all one way, then a new layer going perpendicular. Make 4 layers. Make sure you milk all the bubbles out. Let dry for 36 =hrs.
5) For the head, do the same for each head halve, using smaller strips of paper. Make sure you milk all bubbles out. And let dry 36 hours.
6)Once the paper is dry and firm. Pop it out of the head halves and attach the halves together with short strips of dextrine soaked paper. Final product is a goose head. Pop the shell you made out of the goose shell mold. Wallah. If you got all the bubbles out you will be amazed at the detail. It will take you 2-3 before they come out looking as good as the mold. Coat both sides with polyester resin or spar varnish for a lighter shell. Goose shells for literally pennies a piece. That said, it takes a lot of time. Sorry.. rambling.