i think that’s a good idea to make a pen for her. that cage is too small to be confined to.
my first thought would be to get rid of the cage altogether, or use it as a litterbox and get an xpen that is permanently set-up. if made square it is 4 x 4 feet, or you could even make it long and narrow so it would be 2 x 6 feet. the xpens come in a few sizes, but the smallest size (24 inches) might be too short. i’d get the 36 inch.
another option would be to construct a pen out of NIC grids. those could be easily pushed aside when she had playtime, but i’d suggest they stay up the rest of the time. if stacked two grids high the height is 28 inches. some bunnies can jump over that.
your final option would be to, again get rid of the cage altogether, and build a condo out of NIC grids. it does take some time and creativity, but you can build in levels so when she is confined, she will have enough space.