You can make your own platform of course; though that might be more expensive (tiered rewards are more inspiring than winner takes all, etc). The existing platforms will allow platform dwellng artists immediate knowledge of the task, and artists that are not already onthe platform will be able to learn about it the same way you might inform those artists about your own platform.
Using an existing place for competition submitions also can get you access to a talented artist who will carry through with the vision of their artwork; For example out of 10 submitions you can have 10 different visions and directions. Using those 10 as a base likely all who vote are evaluating from what is available to see. So, it is also possible to not get the whole range of awesome that a particular artist would deliver given a winning prize; given more time; … etc;
I’d note that @Blindsite2k 's 3D rendering of the forum logo is quite crude; though the vision is excellent and I can run with that, with it I can show a professional the range of the figure that I want rendered and therefore @Blindsite2k deserves due credit in the process of the creative endeavor in Logo Design 3D printed .
Conclusively, some kind of structure should exist; it would be epic to make a new platform for running this contest on the testnet for example with a digital contract and voting system, though I think the prize or prizes would also have to be conference worthy to attract artists to this platform 