best way is indead to remove the dash, but it can be done without. if you go down the without removing dash route you will need 1 person in the car at all times with a squirty bottle of water as theres loads of foam and sound deadening that will smoulder a bit. make sure you pull the carpets back and tape any of the cars loom up and out of the way.
you cant buy replacement panel for this bit, so you have 2 choices
1 cut one out of a scrap polo
2 make your own panel.
it is a bit more difficult to weld in without removing the dash, but there is just enough room. i would advise removing the bonnet to give easier access though.