I suppose you could bodge it with some tie-wraps or something, but most people use some sort of spacer and screw them into holes they've cut in the case. rememeber you'll also need a gap between the rad and the case for airflow.
You will need to secure it somehow as if it gets knocked off you're probably going to get wet.
EDIT: I just realised you're talking about a res, not a rad.. doh.. Similar thing applies though, just without the need for airflow.