Not sure about copyright on your country, but here as far as you aren't earning any money out of your work nobody will hunt you. You should claim somewhere on the credits you are doing your work to be shared for free and for pure entertaiment and no legal rights are intended to be broken, something like that (I suggest to inform yourself properly on what exactly you need to write).
If you would intend to sell your mod then you couldn't do it, but as long as it is pure "fan work" they cannot charge you anything, the more they could do would be to prhibite you to use the material, but that's all. For example some franchises prohibite fans to write fanfics (not sure which one, right now, but there are some) but others like Harry Potter and the like are full of fanfics and fan works everywhere around and no legal problems arise as long as they stay on the fandom league. Some people also make shortfilms using movie soundtracks and as long as the films aren't exhibed for economic benefit nobody will prohibite the use.
Besides, I assume you are already using Milius' and Cobb's designs, and I don't think those could cause any problem anyhow.
Anyway, I repeat I am not expert on legal rights and maybe someone else can give you some more accurate answer.
PD: another extra strategy would be not signing your mod with your real name, but using a nickname so you cannot be hunt. This way you are also stating you aren't even using the mod for personal promotion or anything. But that's your personal choice, of course.