I love my large ironing board but didn't want it have to wash the cover. So... I cut up a bed sheet top, slightly larger than my board and then serged it all around. Actually I got 4 pieces from one queen top. I just use a few pins on the edge, slightly underneath, to secure it and then just change for washing. Works great for me!
For cushion, I used 2 layers of batting.
My DH used 1/2 inch plywood for my 24 inch by 60 inch board. It still bends a little. I think 1/4 inch is not quite strong enough, just my opinion. You will love the extra room you get using this board. Be sure to put rails on the bottom so is stays securely on you actual ironing board and doesn't slide around.