I made a quilt with a charm pack and added a large focus square in the center of the quilt along with borders. The pattern is free. The link below takes you to the post I made on the quilt board with a photo and pattern name. Have fun!

https://www.quiltingboard.com/pictur...-t267521.html?

Raid the stash/ scrap pile. Make some more blocks. Add a border.

Sadly, I don't have a stash since I am a newbie. (so far, I've only pieced tops from kits made at my quilting group, this is my first time attempting a "real" quilt) I did buy some white charm squares, so I may be able to integrate those somehow. If not, I will run out and but something today. :)

I usually add about 6.5" to each side for a quilt the size you are describing. I like a larger baby quilt it can be used much longer and can be carried around for years.

Mine always end up square - the one I'm cutting today will be 44x44 because my batting is 45" wide.

After a successful fabric shopping trip, I am set to begin!

I would add some borders. I make quilts that are about 40 inches square. That allows me to fit the top on the backing without piecing the back. That is where my laziness kicks in. When the quilt is finished they are about 41 /42 square with the addition of the binding.

https://villarosadesigns.com/products/galaxy
This pattern looks like it might suit your needs!

All the baby quilts I've made have been close to 45X60. I buy that size batting whenever I have a coupon from Joann's so I always have one on the ready.

Most of the baby quilts I make are between 36" - 40" by 45" - 60. That way the quilt will be big enough for the toddler the baby becomes. I love all 3 of your backing choices, but personal preference is to use batting.