Read through :
https://ludens.cl/Electron/trafos/trafos.htmlNote, that they insert the E plates from alternating directions. That is, as in most transformers the aim is to make the leak inductance as high as possible (i.e. the current drawn by it as small as possible) - the alternating directions help reduce the effective gap between the E and I parts
In a choke ballast (that is what i'd recommend starting with), the aim is to make the inductance (the same one, but not considered as a "leak" in this case) not as high as possible, but instead of a specific value (i.e. to get the correct current for the lamp). So, there is a desired gap size. All E's are inserted from the same side, and there is a gap between them and the I's
The gap can be made as some layer of non magnetic material inserted inbetween (then it's effect is doubled, as the flux have to cross it twice), or using special E's with shorter center leg, or special I's with indentation in them. The latter was and still is one way used in actual ballast production
The more modern approach used by ballast manufacturers like Vossloh Schwabe is, to use an E with short center leg, and shorter I that fits into the E completely, which then is pressed in until the gap is of the exact wanted size (it is pressed while the ballast is connected to a measurement circuit, which determines when to stop). I dont think that the "short" laminations are generally available for purchase, and they would probably be too damaged to reuse if taking them out of a ballast