My husband built one to put over my ironing board. It is fine; not unstable at all. Cost was low. Mine is removeable in case I want to take it off to use the regular ironing board. I have learned, however, to have my dh take it off and put it on again for me, as it is a little on the bulky side and heavy side to do it on my own. Mine is covered with Warm and Natural, then with Duck cloth purchased from Walmart (all stapled to the board). The underside of the board has lengths of 2x4's nailed on so the board fits snugly onto the nose and sides of my ironing board. Mine measures 21"x54", but you could make it almost any size you want. I would imagine the wider it gets, the less stable it is. 21" was wide enough for my purposes.
There are some websites that show how to make one:
http://www.instructables.com/id/How-...d-Into-a-Quil/
http://www.minneapolismqg.com/2013/1...ing-board.html
https://mulberrypatchquilts.wordpres...ard-extension/
http://wildonionstudio.wordpress.com...-board-topper/