I think rabbits inherently have the desire to get into as much trouble as possible, especially when it comes to possibly hurting themselves. Olaf has jumped up and over a 3ft. couch arm in order to escape his early x-pen wall and has proven that he doesn’t mind hopping out of the second floor of his current c&c (cubes and coroplast) cage, including at one point getting caught up on tehe edge and dangling from his back legs. (I know, I know. It was horrific but he was miraculously unscathed – I am now a hawk for keeping the lid closed.)
Anyways, if you have an open-top pen I would suggest covering it with a blanket or tarp so he doesn’t think it is open. Some places have oversize covers with elastic around the edges like a big shower cap to stretch over the top. If he is still too daring and tries to escape that, I think you need a tougher cover like a screen or grids tied together with zip ties.
The last, but most costly option, would be to make your own cage out of storage cubes like others have on this site. You can make them as large as you want and use the same grids to make a cover and doors. Definitely look them up for ideas.