Welcome! We love seeing pix of your bunny (and cat) to get a good idea of who they are.
First, let me suggest you check out the Gallery of pix and videos on this site to get some great ideas of other habitats that folks have made.
My next thought is to go online and price out the XPens that come with covers. I have seen them for sale in several places, usually stores that sell them for dogs, and they have fitted plastic covers/floors that can be purchased. I would think that would be secure.
Another option you could make would be to take some screen or chicken wire and fit it to the top of your xpen shape, being sure to smooth or cover (duct tape?) the cut edges to avoid cutting either the cat or the bunny. The issue for all is getting yourself in and out of the cage once you have the top on – you’re limited to the door.
A sheet of thin, lightweight plywood might just set on the top with clamps that could be removed easily.
Big box pet stores are going to have sales now that the holidays are over, so before I go to Craigslist (used and risky) I would check out local stores for some of the bigger cages. You can have a hutch indoors, just don’t get anything with a wire floor as that is harmful for bunnies.
The favorite home remedy for an xpen cover is a sheet held with clamps. But I can easily picture a kitty squeezing under one, or tearing through it in a jiffy so it’s my last suggestion, not my first.