I've pieced minkie for backings lots of time- a 'bump' has never been an issue- neither was nap- I bought 6 yards of minkie, cut it in half (2- 3yd pieces) sewed the salvage edges together using a 1/2" seam (inside the salvage edge) then trimmed the salvages off. opened the seam (I didn't press with an iron- I just smoothed it open with my fingers). the bulk was not noticeable when the quilt was quilted.