I know about the various locking skewer offerings but something cheaper with 15mm wrench flats would be nice. Ideas?
Longer axle and nuts.
If I understand your question correctly, what you are asking for doesn’t exist. Nutted axles are not skewers, though you can replace a QR axle with a longer one which will accept two 15mm nuts (as others have said).
Why change it, though? A QR locks a wheel as well or better than a nutted axle. Unless you’re planning on racing track, I don’t know why you’d need one.