If in doubt, a jigsaw will fix all
600x280 will fit in behind the bumper, just. Your foglights won't survive either. 500 wide will look a bit silly cause it's smaller than the opening
I think my last one had a 600x300 and it fitted fine, since you have that bottom lip on yours it'll be sweeeeeet
That said the bumper reinforcing bar might not like it, better to maybe go wider but shorter if given the choice. The top 100mm of it is obscured by the bumper anyway. You need to trim away a fair bit of material but nothing drastic
Have a look at how AJT did his, could be an idea? Wish I'd done mine the same, it's very subtle