You could use foam board or a piece of dry erase board cut to size. Check with a framing shop and they can cut mats to any size.
Are you only using that specific size? Honestly, I vote for the creative grids ruler. I use the 20.5x 20.5 CG for squaring up quilts and borders. Easy to use the other marked increments even if I don't need one that big. This is my favorite brand of rulers and I use several different ones.
Consider putting that $25.00 to the price of a larger ruler. Perhaps find a site and sign up for email discounts when the rulers go on sale. Hope that helps!
Good luck with your rulers!