Many people wanted me to share my work with nebulae but there was a reason why i didnt do that earlier.You need to have a powerful computer and AT LEAST 2k resolution otherwise they will not work/look properly. Another problem is the limitations of the SE itself and its "small" skybox for nebuae, plus fps will drops heavily and in worst cases they can crash your game. Finally some of these nebulae were not meant to be viewed as a whole 3D model but rather like a 2D image.
For those who still want these files i will upload them here every time i'll come up with something new.
Download the files and paste them into your SpaceEngine>Data>Models>Custom folder.
(set your "stars" magnitude to the minimum of 15,5)