Does the tank have those little glass ledges either side? If so go to a local glaziers and get a piece of glass cut for the top. Barely changes the appearance at all and then you can keep whatever you like in it
Again i think most fish will jump given the opportunity, for example imagine a fly wizzing around just above the water level, wahay food, jumps and keeps on going"mum look i can fly"opps to far"ok now iam like a fish outta water "
however as said i have found since adding duckweed to my tank's fish tend not to jump so much as before, so that could help you, i would go for the idea from ellena, i myself hand a old 2ft tank sitting doing nothing so i cut it all up removing the cylicone and used that glass for sliders, worked a treat and was free