I'm in the middle of drinking my first keg fermented beer, straight from the fermentor. I also used a spunding valve and had zero problems. Beyond spunding valve and a prv, some measures you can take are:
- leave lots of headspace
- get on the higher side of pressure, this will further lower the krausen
- you can get one of the 1.5 inch tri-clamp lids for a corny keg and fit it with an additional prv and/or spunding valve. I plan on eventually trying that.
That being said unless you overfill the corny keg, I don't think you have anything to worry about. I used 15 liters of wort and didn't have any issues even when I opened a prv and krausen quickly started rising. It still didn't blow out.