Well, often a dumped litter box and a mess is their non-verbal way of communicating that they are unhappy about something. Probably related to the litter pan or housing.

I’m thinking she may feel that her dog crate home is too confining, and wants out. Have you considered getting her an X-pen that you could set up that would give her more space to roam and get comfortable in? If you go above and look at the habitat section under Bunny Info on the green banner, you’ll see some very creative housing. You don’t need to build anything too elaborate, but my guess is a big bunny wants a lot more space.

That said, a bunny may also toss litter if she doesn’t like that type or brand. Did you ever change her litter? My bunny surprised my by tossing the soft type and preferring the pellet type. I knew this, after spending on sample bags, because she stopped the tossing once she was pleased.

Finally, you can buy a litter pan that comes with a screw mechanism attached, which I use, that goes outside the bars and hooks onto the pan so it stays put. It’s a corner pan. I got it at a petshop. Also, if you just want a plain litter pan, poke a hole into the side of it, not far from the top, sand down the rough edges of the hole for safety, and run a tie through it. Hook the tie around the outside of the cage bar to hold it in place and to make it available for you to untie when it’s time to clean.
Some folks use dog clips, but I don’t exactly know how to clip a litter pan with one. It may be easier than a hole and tie if you can find one that fits.

Here’s a rectangular one with the lock visible. Actually, I could use a fresh one. I may try this!

I agree that she would be better in an x-pen for your big bunny and you can attach a litter box to the side – the ferret one is too small though but the idea is to attach it to the side – I have used a bungee cord to attach before – need a longer one – you want to put it around the litter box and attach the hooks on either side of the corner of the x-pen (I hope that makes sense) otherwise if you tried using zipties it would be too hard to change the litter box.

Some rabbits are diggers too that’s just their nature. Try buying a litter box with a grate.

Here’s some photos of my x-pen set ups for my big bunnies.

I agree that you might want to look into an xpen. For such a large bunny they need more room than what a dog crate will provide. The small critter litterboxes are going to be too small, so you can use a regular cat litterbox and like Pam said, drill a small hole near the top. Loop a zip tie thru loosely. Take a dog clip (you get dog clips with the xpen so you could use one of those) and put the ziptie loop inside the mouth of the clip and clip onto the xpen bar. If you position the box in a corner, the two sides will help to anchor it in.

I use this method to secure a hay box to our xpen.  Not the best quality pic, but you can see what I mean by the ziptie loop.  This way you can clean the box, just unclip the dog clip and you don’t have to cut the ziptie every time. 

I use xpens. My French Lop especially needs a larger area. I’m not sure how much Sara Jane weighs 15 pounds?