Michael's sales some different types of picture framing mats. I've used that before with great success. It's a little tough to cut, but it works well. The picture mat will come apart if try to pull it apart, but once it's glued in.....well it does a great job.
You might also try making your own paper ply (plypaper?). Start with a couple sheets of 110# card stock or poster board. Remember that, just like wood, paper does usually have a "grain" to it - it will tear easily and relatively straight along the grain and not against it. It will also roll easier along the grain than against it (one reason I always design my models with the tubes oriented top to bottom).
I'm not sure how thick posterboard is, but 110# card is .009" to .010" thick, so you can judge by that how many layers you want. The next trick is to alternate the direction of the grain, just like in plywood. Not only does this make the plypaper stronger, it also reduces curling (the paper will want to curl across the grain). If you like, you can even rotate the paper 45 degrees for a couple layers to make it really strong, but that isn't really necessary. Then, it is just a matter of gluing layers together until the ply is as thick as you want. I like about 6 layers - that gives a ply from .054" to .06" thick. Eight layers would be about .072" to .080". Remember to keep the glue layer thin (I like to use an old credit card as a squeegee) and press the end product under a couple of heavy books until dry. Once dry, the stuff should easily be as tough as the fin stock that Centuri used. When building ply, try to stay with an even number of layers - that will help control the curling the best.
One caveat - If you use white glue, don't leave your rockets in a hot car for very long - the glue softens with heat and the fins could de-laminate (I've had some do that). Yellow glue might very well be more resistant to that, but I haven't tried using it. If you do, spread it thing just like white glue - treat it just the same as white glue in making your ply.
By the way, if you don't have a digital micrometer, you might want to get one - it is a really useful tool for rocket building. If you have a Harbor Freight near you, they have them pretty cheap. With a micrometer, you can customize your ply to whatever thickness you want, can measure other fiber fins to see how thick they should be (the Estes Viking is pretty much the same fiber stock that Centuri used) and even try other papers/cardboards.
Sometimes you can pickup scrap matte board at a framing shop. They might have leftover ends and edges for very little money.
I just finished making some fiber board fins for a Vector V clone. I followed the Eric Truax method of making ply fins.
I usually make 3 layer fins but this time I did four. The glued ply was: 1 layer of 110 lb. cardstock, 2 layers of cereal box cardboard, 1 layer of 110 lb. cardstock. The fin was very close to 1/16" thick.
If you use the cereal box material, be sure to rough up the printed side with sandpaper so the glue will stick better. To seal up the edges run a bead of glue over the exposed layers. That should help prevent de-lamination.
That stuff is very light and has a lot of stiffness. It is easy to cut and work with, and to mount a panel on the end of an arm (like on the satellite killer) you can slot one side and seat the arm inside the foam core. If you don't like the "look" of having 1/4 inch thick panels, you can easily bevel the backside around the edges to make it look better.
yes but useing the foamcore on the scatalite kill is just wrong lol.
I stoped by a fram shop in my area and they gave me a full sheet of matt for free and have plenty more they said as no one orders this particular black might go back in a few days and see about taking the lot off their hands. I will be starting on my scatalite killer tomarrow
Matte board is great stuff too, I was just trying to suggest an option.
I have found that matte-board needs a little protection around the edges. After you cut your parts, while the cardboard is still nicely and neatly bonded, it helps to soak the exposed edges with thin CA. You may need to do a bit of touch-up sanding afterwards.
Otherwise, matte-board seems to like to unravel and fray at the edges, and you can end up with a model rocket that looks kinda ratty after a while.
You could try using presentation/illustration board. I found three 20"x30" sheets at Staples for 13.00. It's 1/16" thick, and very stiff. I used it to make the centering rings for this motor mount. The rings are not perfectly round, due to my hand cutting skills (lack of), but they work fine.