I don't think there is any part of it that is so thick as to need infill. The threads are a match and I can easily swap parts between printed and bought. They screw together smoothly and securely. The end of the cap is 2mm thick, I'm not sure if thinner would be a good idea. The lip between the end of the tube and the bottom of the base is 3mm but for compatibility the total length of the base can't be shorter than the original or an original cap will be too long to use with it. For the diameter you have to take into account how deep the threads cut. Making the cap shorter would be OK. it would still work with a Estes base. I haven't weighed them, but I printed mine in PLA+ and they feel lighter to my hand than the originals.

Minimizing the weight would require abandoning Estes compatibility. Then you can use a thread that doesn't cut as deep and reduce diameter, and shorten the length of the cap and the base.