Or glue spacers in the fan case holes to make up the difference. Then again, if you've had this much trouble finding these fans locally, you probably would have an even worse time finding various size spacers and other misc hardware.
Just Glue it in - if the surface is flat, then doublesided adhesive (mylar) tape is good. Hotmelt or the "Silicone-rubber" glues will work well on uneven surfaces. My harddisk is "taped" in.
One dab on each corner should do it. (By the time the replacement fan wears out, the computer has been donated to a charity shop and written off in tax).
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