Think in Terms of Force, Not Gaps
A Flemish Giant changes the escape-proofing question completely. This is a rabbit with the body weight of a medium dog, powerful hindquarters, and a calm confidence about leaning on things. She will not slip through bar spacing; she will move the barrier itself. Lightweight playpen panels that contain a small rabbit simply travel across the floor when a Flemish Giant puts a shoulder into them, plastic corner connectors snap under her weight, and a pen that is not fixed to anything becomes a pen she wears while walking.
Start by anchoring. Fix panels to a wall, a heavy wooden frame, or a solid base so the perimeter cannot migrate. Replace plastic corner joints with metal connectors or bolted brackets at the points that take strain — the corners and either side of the door — and test the finished pen the way she will: lean your own weight against each panel at rabbit shoulder height and see what gives.
Doors That Hold Against a Full-Body Push
Her door must fasten at two points, top and bottom, because a single central latch lets the bottom corner of a door flex outward when she pushes low, which is where rabbits push. The fastening itself should be a bolt or a clip that closes positively, not a hook resting in an eye, since vibration from her thumping and barging can walk a loose hook free over an evening. If the enclosure has a lid or a top door, remember she can lift with her back: a light grid laid over a pen is no obstacle to a Flemish Giant standing up under it, so tops need fastening down, not just laying on.
Choose fastenings you can close firmly one-handed, since you will often be carrying hay or a water bowl, and tug-test as you leave.
Height, Hopping, and the Furniture Inside
Flemish Giants are not agile jumpers compared with small rabbits, but they are tall: standing on hind legs, a Giant reaches surprisingly high, and a pen height chosen for a standard rabbit may be little more than chest height to her. Give the walls real height, and keep platforms, houses, and hay racks away from the perimeter — a sturdy hide becomes a step that converts her height into an exit, and she will use it without hesitation.
Any ramp or platform must bear her weight without flexing, because one that collapses teaches her that barriers move when pushed.
Floors, Digging, and Outdoor Runs
On the ground, her risk is digging combined with weight. Indoors, protect the flooring under and just outside the pen, because a rabbit who digs at a corner where the pen meets a rug can work bedding, rug, and eventually her own body into the gap she has made. A solid base sheet that extends beyond the pen walls keeps both the floor and the perimeter intact. Outdoors, walk the fence line of her run looking for the shallow scrapes that precede a tunnel, and sink a barrier or lay mesh under the turf along the edges, since a Flemish Giant digs slowly but shifts a great deal of soil once started.
A Perimeter Walk Built Into Her Routine
Make a slow circuit of her enclosure part of the daily feed: push each panel, tug both door fastenings, glance at the corners for digging, and check the ground along any outdoor edge. It takes a minute, and it catches the connector that cracked yesterday rather than the escape that happens tonight. Keep her in a properly closed room or run while you clean, rather than trusting a temporary barrier of boxes, which a Flemish Giant treats as furniture to be rearranged.
Her size makes her easy to spot but hard to catch in open space, so prevention is worth the hardware, and if she ever does push her way out, KinsLe's Lost Pet Alert System puts her description in front of the Circle of Love nearby, giving the person who spots a very large rabbit in their garden a straight route to her owner.