kicking litter out of litterbox

So I have had my adult bunnies for almost 3 weeks now and just this past week they are kicking the litter out of the litter box. What a mess all over the floor. So I went from having a regular litterbox to taking a plastic tote without the lid and cut a hole on the side, I thought I solved the problem but those little stinkers are still kicking almost all the litter out of the hole. I havent seen them in the act and I'm sure their having a blast while their doing it. Any ideas on what to do ?? The litter I'm using is that paper litter, like i said it just started up till this past few days they were alot neater with their litterbox.

I put the litter box inside of an apple box--the kind with a top to it and cut a hole fairly high up as it's one of my younger buns doing the same. She stopped digging it all out and uses it as a place to hang and chill too. I also have a giant litter box full of hand shredded newspaper for their digging needs as it's easy to pick up and toss back in. Some just like to dig, the little buggers!

I had a similar issue for the first week I had my buns. So I went and got the REALLY high sided cat pans. Its atlas 6 inches all the way around, and about 8 inches in the back. My bunny is only 4.5 months and she gets in and out with ease, and I have yet to find any litter on the ground. Unless your adult bunnies are over 6 years of age and struggle with hops, I think you should drop the hole. They don't need it. They will hop in an out, especially if you have the hay/hayrack (which I do) in it.