bpp1 = bpp0 + [ 0, mainwall_th - behind_recess ];
lppA = bpp1 + [ seal_th, -recess_gap_end ];
lppB = lppA + [ lid_edge_th, 0 ];
-bpp2 = [ lppB[0], bpp1[0] ];
+bpp2 = [ lppB[0], bpp1[1] ];
bpp3 = [ bpp2[0] + (bpp1 - bpp0)[1], bpp0[1] ];
bpp4 = [ bpp3[0], bpp0[1] + mainwall_th ];
lppC = bpp3 + [ 0, -recess_gap_end ];