I had so much fun making my own personal lined paper that I thought I'd make some grid paper while I was at it. Here is the outcome:
The squares are larger than the with standard Filofax version, which suits me. If it suits you too, then you're in luck.
You can download the pages either as editable Word files or as PDF files. If using the PDF versions remember to set your printer settings so as not to shrink the pages.
Download the pages as a Word file or a PDF file.
Each file should be printed double-sided with the duplex set to flip along the short edge. You can then cut out and punch individual pages as outlined in this post.