how to apply a canvas 'grid' to leaflet without curvature

I'm trying to create a grid of tiles in canvas to represent some data

the original data comes with UK OSEAST1m and OSNORTH1m, i'm the grouping the data up into aggregates of, say 40km (as per the image). I'm then taking the grouped data and and projecting it - from the grid ref to lat/lon using geodesy, and then creating a rectangle in canvas.

It works roughly, but this means that there is a grid, as such, but squares that roughly follow the curvature of the projection.

How can I make a grid, so that the squares are aligned?