Plasterboard patching

I need to patch a shaver point cut out in a plasterboard wall (never use one so decided to get rid)
I've got a piece of board and cut it just bigger than the hole. I gather I should fix this to the plasterboard wall behind the hole
My question is - should I use plaster to fill it or will polyfilla do as good a job
I'm worried that the finish will look different and show through once painted
Any tips for this sort of thing appreciated
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