Larry- without knowing exactly what a Sentra's roof looks like, here is something I used on a Geo Prizm.

Make 2 "cradles" with a 2 x 2 piece the width of the car roof with 2 triangles of wood on top of the 2 x 2 set to the width of the kayak (hypoteneuses facing one another). Various ways of attaching them, I used flat wood to span the 2 x 2 and the triangles on one rack and flat bar on another. I put some pipe insulation across the top of the wood for padding. At each end of the cradles you make your posts or legs to sit on the roof of the car.
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I angle the ends to fit the slope of the roof and glued some rubber on the bottom, although the plain wood doesn't mar. The posts are attached using an L bracket on the underside and a flat bar on the top. This flat bar extends out about 2 inches with a hole in it.
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Run a turnbuckle through this hole and on the other end of the turnbuckle attach a rain gutter clip. I haven't seen these in stores recently, but I have some that fit the modern gutters without the lips. Connect the two cradles with 2 x 2s to form a big (or little) rectangle.
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On a small car there isn't much of a span, but with those turnbuckles cranked down and good straps, I've never had a boat shift even at freeway speeds. I seal the whole thing with Thompson's deck sealer. Won't win any beauty contests, but for 20 bucks in supplies and not too much time, I'd put it against a Yakima or Thule any day. They're easy enough to just make one for each car you have.