Random thoughts...
First, given your setup, you don't have many places to hide something. Just an observation.
There are a couple of interesting things to me: your colleague and the altar.
Suggestion 1:
Have your colleague tell you how to get the contraption. How you get him to tell you, I don't know. But he would tell you to do something like "go to room X and do this", where "this" is something a person might not think to do. Or ever think to do.
Suggestion 2:
Hide it in plain sight. Don't call it a contraption. Maybe it appears as a chalice with odd runes markings on the altar. If you take it to your colleague, he tells you "Hey, you found the Fulgurator! If you press the runes in this sequence, you can upgrade your lamp." etc. Or you place it inside the lamp with the light source, that sort of thing.
Suggestion 3:
Have a book on the altar. Reading the book, you find it contains descriptions of/prayers to the various gods who could be prayed to at that altar. One of them is the the "god of light". If you pray to this god (by name) at the altar, then the contraption falls from somewhere above.
Off the top of my head (well, after some thought)...