I missed the part that it was a bike seat, focused in on double loop lacing. I agree that on a bike seat round holes are the way to go. Also, double loop is bad, Mexican round braid is good. Almost always when you are using Mexican round braid you use round holes.
I do not agree though about the backside issue. I will post up a few pics showing the backside of a couple projects. I have something in my possesion that I am relacing (not my origional work) that round holes were used on so I will have the backside of how I lace and how it looks when done with holes. The "cleanness" issue is just as big of a deal to me on the back as on the front, To me, slots always win.
Also, ALWAYS use Kangeroo. If you want your work to last, use Kangeroo. Calf lace is good for practuce and not a lot more.