There is one near me in the countryside that is oak I think, with a sloping roof piece that is an inch wider all round to deflect a bit more rain that has survived 20+ years.
Our Men's shed made some painted signs of waterproof ply with oak edging, well painted, for a local woodland. Within a fortnight they had been vandalised. Repaired and remade them and put them much higher up the trees.
We made our own sign for the shed which is in town in the same way so the vandals set light to a car beside it.
We remade it in 4mm aluminium so they bent it, so we have no remade it in aluminium again with a 3/4 inch wood backing. Fingers crossed.

I'm with Speedybuilder on that one. UPVC lockable and ready made. South Yorkshire Windows on Broad Lane, Sheffield, might be worth popping into if you're in town.

For the backing board, I wonder about MDF. Before everyone falls about laughing, it's possible to get a fully waterproof grade,rather than just moisture resistant. Yorkshire Wildlife Trust have built a viewing screen out of it, fully exposed to the elements and untreated. As a test, they left a piece soaking under water for several years.

It's easy enough to cut, and low maintenance. It could easily be faced internally with something softer for drawing pins, or a piece of steel from the side of a washing machine. That's slow to rust and would allow posters to be held up with magnets.

Bill

Edited By peak4 on 18/12/2019 18:57:36

Some advice borne of experience.

The best location is fixed to a wall.

Put it somewhere visible at a distance, although it will attract more attention, it will be less likely to be vandalised than if in an out of the way place.

Glaze with polycarbonate, it scratches more easily than acrylic and may need to be replaced sooner but is less likely to be smashed.

Make sure that the geometry doesn't allow the glazing to be pushed in far enough to pop out of the glazing bars.

Don't put it so high up that kids or people in wheelchairs can't read the contents, or so low down it is easy to kick.

It will get condensation inside even if it is 'sealed' so make sure it is ventilated with drain holes at the bottom.

If on legs, bury them at least 60cm, if not metal or hardwood make sure they are pressure treated and UNCUT after treating below ground level.

Make sure there is a system in place to remove old notices and add new ones. Make sure the locks are quality and reliable and that it is easy to open and secure.

Include a QR code for your website. Relevant attractive information that addresses all potential visitors is least likely to attract vandalism.

Materials, I would go with quality hardwood, then good quality metal as preferred materials.

Neil