May be prudent to weld a bit more thickness and Just have the 5mm skewer pass through the outer Most hole .
So wheel wont Go flying out on its Own .. Putting the skewer thru the hole and the axle and out another hole will be a simple safety feature .
so 9mm axle sits still in the dropout to support the weight via the axle ends . another safety system may be arraigned,
but you need to consider the forces a disc brake exerts wanting to push axles out of the dropouts.