You've got a tiny little rabbit, and a 36" ex-pen. You don't have to worry, right? Hah!
Rabbits can jump at least 2 feet, and often up to 4 feet! They also can use those back feet to "push off", much like a cat making a big leap, and get even more distance. Amazing little athletes!
So what is a poor human to do? We worry about them escaping and getting in to trouble - maybe even danger - when we are not around to watch out for our little buddies. There are a few things you can try to keep your Houdini safe and in that ex-pen.
First, the home remedy: you can take an old sheet, stretch it taut of the top, and use clothes pins or binder clips to attach it to the top of the ex-pen walls. You'll have to use a good number of pins or clips - the little devils know if there is a weak spot in the defenses, and will still make it over the top.
If you'd like to get a little fancier, or you aren't having success with the sheet solution, you might want to try an "official" cover - one made specifically to use as a cover for ex-pens. When searching for them, try using the terms "shade" or "UV", as these covers are often used to keep sun off dogs in ex-pens outdoors. Here are some examples from online retailers:
Notice these covers have all kinds of loops along the edges to help you secure them to the pen - it looks like the orange ones have even more loops, so would be preferable.
You may end up with a disapproving bun for a bit, once those escapes are thwarted, but better mad than hurt!
Good luck, and let us know if you've found any great solutions to this problem, so we can share!