Fingerprints would also require proper forensic equipment, as they're rather hard to see with the naked eye unless something is particularly dusty.

But what would I search for? Torn fabric, fallen hair, blood where they may have scraped themselves while constructing the machine - all of those would be adequate and easily missed by the culprit. Furthermore, while they may lie about their knowledge, unless they're completely dedicated to their alibi it's unlikely they would be able to conceal all of their skill in something.

[ Which in any normal circumstance would be enough. But Furudo Erika is extra as shit, y'all. ]

But why stop there? The circumstances leading up to the murder would also change what evidence I would look for - what was the motive that week, if any? The pieces used to construct the machine? Is there anywhere in the vicinity where the culprit could find the spear, or did they have to make one themselves?