I gather that you're trying to put a QR wheel into a fork who's dropouts were made for a 5/16" (8mm) solid axle. The proper remedy is to file the dropouts open.

Assuming the fork is square, and you want to retain that, file only from one side of the dropout slots keeping the other as the reference. Use an axle or bolt as a gauge so you only enlarge it enough to do the job, and do not file the tops of the slots.

You could file two flats on the axle, but that's a pain in that you'll need to keep orienting the axle every time you mount the wheel, and there's no benefit to leaving the fork alone, unless you plan on going back to a 5/16" solid axle later on.

If you totally lack confidence in your filing skills, than your only option for using that fork is to go back to a smaller solid axle, since QR axles don't come small enough.