My first thought was to place some kind of plate against the inside of the saucer, then fill the hole with ... whatever was going to be the next step. I used a lid of a Chinese takeaway-box, easily cut to shape.
I read somewhere that you could melt down sprue (we all have LOTS of those) and pour it into a receptacle, so I traced some tin foil to make a kind of mold and started heating sprue, cut to little bits.
I am now thinking about other options, which include some kind of clay mould, in which I'll pour a resin-mix. The other option is finding out how to work with styrene and cutting it the right shape. Resin will give me the possibility of actually making a mold with the right hull-detail : I'll simply cast a mold from the other side of the saucer. Well, "simply" ... I have yet to discover how that'll turn out.