If you have a powered mitre saw there is no reason why you should not be able to make good scribed joints at the corners, especially when you'll only have one scribe per corner! The hardest part to cutting the scribe (pre power tool days) was cutting a good enough mitre (by hand) to enable you to get your scribe outline. Whatever you do don't use "Pollyfilla" at corners- if you need to use a filler use a small bit of flexible caulk, this won't shrink, crack and fall out of the joint over time.