two fit on a car’s rain gutters, but can be rigged to fit some factory roof racks. one of these accepts yakima crossbars (not included). the other accepts bolts for whatever you like.
the third set clips on above the doors and accepts bolts.
i also have two sets of homemade canoe racks/ crossbars. these are 2×4’s that are painted and padded and can be bolted down to the towers. one set it predrilled, the other is not. i have two bike rails that can be mounted onto these crossbars. they have the strap for the rear wheel, but not the axl clamps for the front forks. you could pick up a set from rei or get cheaper ones from rocky mounts.
$20 for a set of towers and a pair of wood crossbars.