Haha!! I thought it looked odd with a gaping hole on the drive side.
Is it possible to plug with something ? Like a cap marching the one on the non drive side crank arm ?
Not sure you'll find anything matching. I spent ages looking for a replacement non drive bolt for xtr m970 cranks and came across a set that included a blanking cover for the drive side. Problem is, the m970 bolt won't fit the m960 due to Shimano going all weird with that bolt arrangement on the m970 cranks alone.
The drive side would fit, but wouldn't match.
You could try making something work with a hope headset head doctor perhaps?