We never get to hear how it was exactly that Barty Crouch Jr and Wormtail managed to overpower him, but we know that at minimum Alastor was intensely paranoid - to the extent that one of his defences was a set of enchanted dustbins that fired rubbish at intruders (something Barty sets off himself).

I've always imagined him sneaking around, cackling like an oversized, peg-legged Macaulay Culkin, setting off one trap after another as Barty and Wormtail cart around an increasingly shrill/angry baby Voldemort as they try to navigate this heavily enchanted deathtrap of a house in increasingly humerous and painful ways.

The only caveat is that this MUST be cannon compliant, so Mad-Eye must be caught in the end.

​

Do your worst.