Can be simple as a can dropped in space with a build cost of around 20 million.
When right clicked will allow the owner/user to see Corp Ad, Join Channel and Set Destination.
^^ This would let corps drop them in rookie and other NP locations as a visual marker to interact with.
Billboards :
The same thing as above but fancier with corp or alliance logo. Say around 50 million
One structure per system per corp should do the trick. Spam like the garbage that these dumb russian, chinese and polish alliances put into space with the depot spam would be successfully prevented. No dropping on behalf of someone. You have to be in a Player corp to drop such a structure.