My planner has a lined 'notes' column, where I draw a grid for my habit tracker by adding vertical lines to the existing horizontal ones. It's a pain, not always particularly even, and I hate doing it, but I have a clear ruler and the little 1/16th inch marks kind of serve as a guide to where the next line should go.

Edit: Also, I've recently started seeing stencils used, I've seen ones for those round/spiral habit trackers but not so many grid stencils.