OK, having thought about it, and with the benefit of some local knowledge, I'd have figured out some way to weight the body and lose it in water. There are umpteen reservoirs in the Pentlands with roads leading up to them. The West Water reservoir is a cracker because it's some way past the last habitation, and yet you could drive there very quietly on a decent road. Do it just as dawn is breaking and there's barely any chance you'd be seen or heard.
The tricky bit would be to get the body trussed up well enough with some heavy weights so that it wouldn't bob to the surface. There are plenty weights to hand of course; these hills are covered in boulders. But if you could do that, possibly adding the boulders right there at the edge of the water, you could probably drop her in without needing any sort of boat. Reservoirs have steeper banks than most lochs.
You could do the whole thing door to door in less than three hours. If nobody knew where or when to look for your car, and you left your mobile phone AT HOME, you'd have a high chance of getting away with it.
On the other hand, I know my way around up there. Gilroy would have had to figure it out from a map which he probably didn't even have.