Idk, the "t" and the "d" sound pretty much the same to me. It's like the Cecily/Sicily conundrum. My accent makes them sound the same. I just think
Alasdair looks nicer than
Alastair.
I thought about nicknames too. I love
Alasdair too much, I think, to use a nn. If other people wanted to call him
Al or
Ally, I wouldn't mind though.