So I'm working up some ideas for a high power rocket and aiming to go for my level 1 UKRA certification at some point next year. I'm planning to scratchbuild as much as possible which means it can be tricky to know the exact weights of items you are designing into the rocket.. such as the nosecone which I plan to 3d print.
I can get a really accurate estimate of what a component is going to weigh by modelling the piece in CAD (I have some code I use in Openscad that generates nosecone geometries based on parameters I feed in) and exporting a .stl file as if I am going to print it.
I then load the stl into the slicer I would use to generate the gcode for my 3d printer "Cura". Rather wonderfully having selected the infill and the material, Cura will give me a weight value of the finished item. I can then jump back into OpenRocket and overide the mass of the component which is then incorporated into the design and adjusts the centre of gravity accordingly. Neat!