It took me a week of jumping here and there in order to find it, and I was (and still am) quite a noob so I thought I would get a reward in cold hard money. Not in terms of the value of the mineral. And I didn't have any mining equipment with me. So yeah. Wasted my time.
Certainly one idea I have is only check sites with enough slots. I won't even look at 1-slot moons. Maybe not even 4-slot. If I don't find it then I'll just keep the mission in cargo until I'm at Earth to avoid the cost of cancelling it.
Ofc, a 1-slotter with a diamond deposit would be a loss to me, but I think that's too rare for me to worry about. Atm, # of slots is more important to me than anything else, since it makes collecting the resource more convenient time-wise.
Problem with having bad prospector missions in cargo is sometimes I forget which is which. The inventory system in this game needs a lot more tools to make managing it easier. A lot of players ignore this and I think sir emi does too. If a different system is planned to be used then get on it. Otherwise add some inventory management tools. Just do something. (For example, having an ignore feature for inventory might work. Or maybe a way to flag certain things as no drop. Or just a highlight toggle.)