I can write down some guidelines and give some examples. Keep in mind we have not yet confirmed how we will be doing a contest. Only that we want to and that this here is probably the best way.
1: Don't go big. The first contest would probably focus on small ships. For reference the largest "small" ship in the TG lineup is a 140 meter combat vessel. Everything in the small category is notably smaller.
2: Build a set of ships. If we need to give a faction some ships, it's better if there are at least a few to give them. Build a set of ships that have the same style. Maybe a few types of fighters/bombers or a small transport.
3: USD where possible. We have 44 meter long fighters in the TG lineup that have USD on them. At the very least we can alter the dock to suite our needs if the doorway is placed on the build.
4: If you cannot fit a USD on the build try to give it a docker on the bottom of the build. The TG shuttle has one. Shuttles and drones are the only ones that will likely need one. These small land-able ships should be no larger than 31 meters long, 11 meters high, and 23 meters wide. That is the dimensions of the shuttle and it barely fits inside the Spawn Station.
5: Interiors need room for crew. Two examples. The freighter has room for 3 crew members. There is a bridge, a crew quarters, and a recreation/kitchen room. The large combat vessel I talked about earlier can support 5 crew members. There is a core/engineering room, teleporter room, medical room, crew quarters, recreation/kitchen room, bridge, and captains quarters. Very RP friendly. Not all of these rooms would be operating at once. Also keep in mind that is about the largest you should go.
6: Make them distinct from what we already have. I will outline the color schemes that I expect us to use.
Trading Guild - Grey hulls. Black details. Blue glass. Even less brown and orange details. Orange tanks. Minimal blue and white striping. Boxy shapes. Blue engines. Very symmetrical.
Scavenger Pirates - Brown, grey and black hulls. Neutral colors. Very spiky and scrappy. If there is striping it will be red. If there is lighting it will be red, orange or yellow. Likely orange engines. The picture below is a good indication of their style. Asymmetrical.
Outcasts - White hulls. Brown trim. Orange striping. Black details. Asymmetrical. Bulky, yet scrappy, like star wars ships.
I cannot reveal the last two races however I am confident that we haven't seen their look around much so I am not worried someone will build in their style. You can of course still use the colors above, just alter which color is the primary hull color. Sorry I can't give more imagery. Really though, go nuts. random factions are completely open to interpretation. Below are two images of the TG and pirate styles.