Why not use your existing over sized grids or buy some used grids for your Speed lights. I went to Ace Hardware Store and bought a rubber "Reducing Coupling" which fits perfectly over my Nikon SB-800's.
This Reducing Coupling is a 4" to 2" in size, the small end of the coupling fits snugly over the head of my SB-800 Strobe. With the use of a small bungie cord you can easily hold your grid in place. The length of the Reducing Coupling places the grid at such a distance that the projected light has a wonderful quality to it. Better than if you where to just place the grid on the face of the strobe.
You can pick up new grids for around $25 each however most grids are sold in sets of 4. Looking on ebay just now show several sets for sale, my guess is that you could pick up a set for between $50 and $75 bucks.