Auto rollback Explicit transactions after X amount of time
You could create a job that checks for open transactions and kills them if they've been open for a period of time and are causing blocking. But you'd have to be careful with this as you could have a large, necessary transaction that takes a while that you don't want to get killed. Imagine rolling back an index rebuild that's been running for an hour on Standard Edition. 

You can use sp_WhoIsActive to find open transactions that are causing blocking, though it'll take a bit of work. 

But I would recommend that you don't do this and instead are diligent with your transactions in Management Studio. Any window that I ever started a transaction in gets closed when I am done. And if there's still an open transaction in it, SSMS will warn me about it so that I can commit/rollback.