This just seems like attempting to prove a negative. Well, not a negative, but a lack of something. A void perhaps!
What if the "evidence" is "We looked, but there was nothing there. Thus, we did not recover any craft or biological evidence." You guys would just claim that's a coverup too, even if that is legitimately what happened. So how will they ever prove to you that there is nothing? Even if every lab and military installation were opened to the public (never happen, but let's say it did), you'd still believe there were secret ones somewhere that they were keeping you away from, so how can anyone ever prove there is nothing?
And saying, "Yes, we saw those lights in the sky that you guys filmed. We don't know what they are, and we have nothing related to them in our possession," is also a legitimate answer that still won't satisfy you, because apparently the least likely answer is the correct one to you guys.
Conversely, it is incredibly easy to prove that something exists by simply producing it. So let's see Element 69420.