Just think of how tricky it can be to time your base and final turns when practicing an engine failure in the pattern... those two 90* turns can really be tough to judge! And those are done usually at a decent clip and a level flight attitude.
I generally do what might be called a "modified" base; that is, a sort of 180-degree descending turn. It could also be though of as an old-school, "unstabilized" approach. Assuming a half-mile distance from the runway on downwind, it's a pretty leisurely turn for a light single.
A 180 is, of course, a no-no when on the upwind leg (unless you've managed to claw your way up past TPA), but if one is careful not to get slowed up too much and/or bank too steeply, or turn too early and wind up touching down too long, the 180 off downwind works fine. Did it that way on my last BFR when the CFI "pulled the plug" just before the numbers, and he seemed happy with that.
No point in trying to keep things square when you have no power- and you may have some other problem that requires a speedy arrival. If you think of this as your "general emergency landing off downwind", it'll serve well whether it's engine failure, fire, or whatever- you'll get down sooner.
Also, if you try to fly a normal base and final, when you're making those two turns, you're now lower and likely slower than you would be if you just wait a brief interval then carefully come about while descending.... not to mention farther from the runway. Not saying it can't work, I just feel safer making one turn, closer to the runway.
I'd rather risk landing long than getting too low on final with no power. Might overrun and hit something or damage the plane, but that sure beats hitting something while sinking below the glideslope, short of the runway. You can slip like mad if you're long, and land hard due to high descent rate if that's what it takes, but we all know what happens when you try to stretch a power-out glide on final... eep!
And why go an inch farther from the threshold than is absolutely necessary to turn around and get it on the runway, when you have no power available to compensate for the effects of turbulence, etc.?
Needless to say, this will be a short-field-with-obstacles-type approach, especially if the trouble strikes on downwind.... and the exact point in time and space to initiate the descending turn will of course depend on runway length, wind, etc.