Sorry if I missed it but what size is this tank? I would be very wary of filling a large tank with a broken or questionable center brace. The center brace is a very important part of large aquariums. If it breaks the entire tank weakens. The tank may not burst or crack immediately. It could take weeks or months for a problem to occur. But I've seen some pretty substantial damage caused by tanks with damaged center braces. As for using a full lid, I would do so. I've seen Oscars and some other fish jump out of tanks trying get food or from just being spooked by something. Many glass tops come with a flexible plastic strip that covers the gap between the back of the tank and the glass top. You can easily cut this plastic strip to accommodate a HOB filter without leaving a big space for a fish to leap from.