Here's Mine. I first selected the saucer and engine and did a seperate/join to get a solid there. then for the dome, I nudged it down a bit and did the same thing to get a solid. I think the issue with the dome is there is a funky space towards the back that may be creating slivers with the boolean. Bringing it down a bit might just have gotten rid of the undesirable slivers. you could try getting rid if that creavace since it will not be needed or seen anyway.