Inkscape is the way to go, at least for me. But, if you are doing, for example, squadron badges or other complicated shapes I suggest to do waht I did: get the best photo you can of the sahpe you want to convert to decal and hire a professional to do the raster-to-vector conversion. There are lots out the on the web. I found one on https://www.fiverr.com I got some images converted for less than $15 (for us living in CrazyTown USA) and then you can reshape in Inkscape.
Ber aware, Inkscape has its issues, specially on Windows 10, and their raster to vector features are still in need of some polishing.