The ones you bought are probably electrical knock out plugs where 1/2" is the trade size but the size of the hole it plugs is 7/8". You need 3/4" hole plugs and not knock out plugs (confusing I know) these are the ones I used part # 9563K45 found on this page http://www.mcmaster.com/#snap-in-plugs/=cc00hm
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