As someone who can never ever manage to get a quilt to be within one width of fabric, size constraints are hard on me!
So here's an idea... if you leave the basic size of the quilt as 3x4 blocks and take off the bottom and one side row, you would only be 4 blocks short of having two quilts. Then you can add that sashing border and a wider border and still be within your size constraints.
This is a sail boat top I made last year, didn't get a shot of it after the binding was done. It is a "big brother" blanket for a 3 year old so he wouldn't be jealous of the baby gifts. It's huge for a baby quilt, small for a twin, but just right for this gift.