I need help with creating endpapers

I'm really struggling with endpapers, since im trying to bind comics and dont have any decorative paper big enough to do a simple fold.

Comics are 6.625"x10.25", and I know I need an endpaper at 10.25"x13.25" to properly make a fold.

All of my decorative papers I have are 12x12 (my wife has a TON for scrapbooks.

Fortunately, I do have a big pack of 11x17 printer paper they were throwing out at work. 24lbs. That would fit the bill as a backing, but i just dont know how to go from point A to point B.

My first attempt at making an endpaper had a really ugly seam where the decorative pages met. I had used some extra linen tape as a fabric spine for the endpaper. It holds nicely, but its just not the quality i want.

Any ideas about ways I can fold the paper to hide the seam? Bonus points if it supports a sewn endpaper option.