pine plywood or cork
If it were me I would look at the box store for some 18" square cork floor tile. If you make it a multiple of 18" or what ever dimension the tile comes in, then replacement will be very easy, no cutting involved when they get buggered up.
I suppose that it will be exposed to some extreme weather, wind and rain, so a durable surface is needed. I don't know about this idea, it just came to me. How about a metal surface with small magnets to secure the flyers? Another idea is to have a flip down Plexiglas cover that keeps the flyers dry and secure against wind. I used this approach on our Private Road Notice board at the beginning of the road. It had a twist knob at the bottom to hold it down, but a magnetic catch would also work. A place for blank paper and some Sharpies may be handy...... :nerd2:
As far as instructions go, it's an easy build for a framing carpenter or a DIY'r with deck building experience and a skil saw or miter saw, probably both. The 2 end posts are identical and a separated by 2 horizontal beams. The roof is 2 identical pieces attached at the top and bottom. Shingle the roof before you attach them to make it easier. It will be heavier, but much easier to work at ground level.
The joints where the frame members cross, are simple half laps where the same amount of material is removed from opposing surfaces at the joint. Use "coated deck screws" for all your attachments to avoid rust. Cedar shingle will make for a rustic look.
Penofin deck stain will make it last through all sorts of weather. So there you go .......