You could buy clear plastic carpet runner and place it under the box.
Gail

Duffy used to have trouble like that.  I had the large-size litter box, but
he would back up and "spill" over the edge.  That was solved when I bought a
large-side hooded box, discarded the hood, and used just the base.  The base
on this type of box is higher than the ordinary litter boxes, and it
completely solved my problem.  (I had previously tried one of those frames
on the large-size box, but that didn't help.)  WalMart sells the hooded
boxes, and so do most of the pet stores -- but be sure to look for the
larger size.

MaryL

Diane,
It might be expensive, but you could try those pads they use on hospital
beds.  They are absorbent on one side, but liquids don't seep through. The
other day, I happened to notice that our grocery store had them, but I
suppose they also would be available at a medical supply place.
Annie

I use plastic trays with low lips under the litter boxes.  One is really
meant to go under a Xmas tree to catch any spilled water, & the other is
from the bottom of a pet crate.  They're each slightly larger than the
bottom of the litter boxes.

Cathy

--
"Staccato signals of constant information..."
("The Boy in the Bubble")  Paul Simon

I think your best bet is some sort of absorbent material on top of a
plastic mat, and of course you have to pay close attention and clean
it up several times a day. Without the absorbent material the urine
will just roll off the mat. One bad effect of using any sort of
plastic or "waterproof" material is that any liquid that gets under it
is then trapped on the very surface you wish to protect. Is there any
way you might be able to find a wider litter box? That might help.

Good luck as I know this is frustrating.

A friend bought a Rubbermaid type storage box because of the same problem.
The box was about the same size as a litter box but the sides were higher.
Joan

Take a 30 gallon garbage bag and cut it open down one side seam, then
cut open the bottom seam.  It will form a good sized rectangle when
opened up.  Put a couple of layers of newspaper on top of this and put
the cat box in the middle.  The newspaper will soak up most of the
urine, all of it if you are lucky.  When the paper gets soiled, you
can roll the whole thing up - bag and all - and toss it in the
garbage.

Sara (been there, done that, with my old cat Timmy)