Start with a queen size mattress pad with the sides cut off (60"x80"). After wash it shrunk to 60"x74". Fold in half and cut; comes out to 30"x74". Use only one half for each pad. (You will be able to make two pads that will fit a 2 x 5 c 'n c cage.)
Lay fleece the wrong side facing up. Measure about 3" - 4" wider than the mattress pad on all sides. You can probably use less if you're good at sewing. The fabric width is 60". I got 3 yards but had a lot left. The actual size of the fleece that I cut out for a 2x5 pad before wrapping around the mattress pad was 36" x 80" (about 2 1/4 yards).
Lay the mattress pad on top of the fleece. (You may use two layers of mattress pad if you wish to absorb more urine or make it softer.)
Fold fleece over the mattress pad and pin.
For round corners, just fold over; they will come out sharp (with round corners hidden inside).
I usually sew 3 lines per side. One close to the cut. the second one in between the first one and the edge of the pad. The third one around 6" - 7" inside the pad so it won't shift when washed.
This is how I sew the line inside. I don't really measure; just guide it along the sewing machine.
Close up of the corner.
Close up of the corner from the back.
Cropped view of finished pad.
My daughter's stuffed kitten is on the finished pad. The finished pad came out to be 30"x74"; fits a 2 x 5 c 'n c cage perfectly.
The pad on the lower level covers a 2 x 5 grid area of the 2 x 6 grid c 'n c cage. The size of the upper-level pad is 1 x 5 grids. With the dark pink pads, I got a 2x5, 1x5, and a 1x2 pad out of the 3 yards.