And in particular he claimed that this showed it had to be Euclidean because humans couldn't imagine it otherwise.

No, he never says that. Feel free to cite something from Kant's writing, or the SEP or something. I may be wrong, but I just read though the Aesthetic again, and I couldn't find anything that would support your claim.

EDIT: I did find one passage that mentions imagination:

1. Space then is a necessary representation a priori, which serves for the foundation of all external intuitions. We never can imagine or make representation to ourselves of the non-existence of space, though we may easily enough think that no objects are found in it.

I've edited my post accordingly, but my point remains the same. Notice that Kant does not mention the flatness of space, nor is it at all obvious that he's inferring anything from our inability to imagine the non-existence of space. END EDIT.

You gave Kant's views about space as an example of someone saying 'because we can't imagine it otherwise, the world must be such and such'. Kant never says this. What he says is that the principles of geometry are not derived simply from the analysis of terms, nor are they empirical. Kant is very, very, explicit...almost annoyingly repetitive, that he is not talking about the world, but about our perceptive faculties. And if indeed we cannot imagine x, that does seem to me to be a good basis from which to draw some conclusions about our perceptive faculties.

I have no idea what Kant would say about whether or not we can imagine non-Euclidian space (I have no idea myself if we can) but the matter is complicated because 'imagination' is a technical term in his philosophy. He thought space was an infinite Euclidian magnitude, but Euclidian geometry was the only game in town at the time.

Anyway he's not a good example. As I said before, I don't mean to dispute the point the example was meant to illustrate. I just wanted to point out that this is an incorrect view of Kant's claims about space. It's not really very important what he thought about space though.