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Since a speedlight head is small and able to be zoomed and in my experience there's not much advantage in using a grid vs. just creating a snoot with aluminum foil or rolled up paper.
In addition to controlling the footprint of the light one of the things a grid does is cut down side spill and the lens flare it creates when lights are placed behind the subject. Below is an example:
I have low ceilings in my studio space and added a grid to my hairlight not to change the footprint (note the circle mask used to do that) but simply to cut down the flare. I got the idea to do that seeing TV lighting set-ups that use grids on boxes for the same reason.
I also have a set of four metal grids for my studio lights from 10-40° which are added to the standard 7" reflector. The advantage there is being able to keep the light away out of the shot, but be able to control the foot print. The way the metal grids for the studio lights control the pattern is by making the honeycomb cells smaller.
If you want to experiment with grids for the speedlight get a bunch of white soda straws. Bundle them into the dimensions of the flash head and wrap in foil. Experiment with various lengths. But if you compare with just using a double layer of foil the same lengths wrapped around the head as a snoot without the straws you'll find the footprint and oveall character of the light similar. Been there, done than