My sister-in-law bought a Cashel fly mask with ears (
here's the one) in a small pony size, I think, and she puts it on over her 34" mini's driving bridle. It fits well over blinkers and everything, and she can snug it enough so no bugs get in around the edges. It's a nice soft material, and not too bulky. Her mini does have a fairly wide head; the pony size might be too big for finer-boned minis, but it is very adjustable...
Oh, and she made the ears smaller so they weren't so floppy.
Myself, I usually just put fly spray or roll-on bug repellent (Swat or Farnam's Roll On) on their faces and ears, but if it gets really bad I have little cotton ear covers I got at the tack store. But they do look pretty silly (I'm not handy with the sewing machine, so the ears are really big and floppy)...
Hope this helps! It's so nice to get out and drive the back roads this time of year, if you can outsmart the bugs!