You can find gridded quilting templates in stores such as JoAnn (where I got mine) that sells a lot of quilting supplies. This is what I picked up - with a 50% off coupon it was $2.14. It's very thin and easy to cut. It's probably about the thickness of lightweight cardstock.
The mat made by Little B is 6 x 12 inches. This template material is 12 x 18 inches. This will give you enough material for four smaller mats (such as Cuttlebug sized) or three 6 x 12 mats if you so desire that many!
For the cuttlebug, cutting mat C is what you'll be using. It's the adapter plate for wafer thin dies. You will cut the mat 7 5/8 in length (longest side) and 5 7/8 in width. Do NOT rely on the measurements on your trimmer. Follow the markings on the template. (Sorry, I should've gotten a full pic - both the template and cutting mat are positioned in the landscape orientation. This is how you'll be able to cut 4 pieces if you want to.)
As you can see here after I cut the material the edge of the template goes slightly over the edge of the cutting mat and beyond the rounded corner.
You are going to trim this to the edge of the outside marking.
Next, you can round the corners. I found that the green paper shapers corner rounder (EK Success) was pretty much the right size. Here's the mat all trimmed and rounded.
Now you're going to cover it with repositionable adhesive. I actually applied it to my cutting plate and one side of the template but you can apply it to both sides of the template. Don't get heavy with it - particularly on the side the dies are going to be stuck to! Your paper will also stick when it's rolled through and you'll want to be able to pull it off.
Here's an example where I used it to make a Polaroid frame. (I didn't have the right size rectangle but it was easy enough to trim the bottom after.) You will notice in the second pic my mat was a bit too sticky in one spot and my paper got left behind a bit.
I just used a damp cloth and gently rubbed the paper off. No problem and didn't take all the tack off either.
One last thing.... it's hard to determine which side is the printed side with the grid! If you find out after the fact like I did - I had a small spot where the paper had stuck and lifted the adhesive and print off in the center - simply remove the grid and flip it over. (Apply adhesive if necessary - since I had applied it to my plate the first time, I had to put some on again when I had to flip the template.)
And be sure you use this with the die's cutting side up. If you put your die face down on the grid you will be cutting yourself some masks/stencils!