As far as I know, you need an invitation letter + form 6.
Unless ATF changed something recently, a match entry form won't do it unless it's an NRA-sanctioned event (or State affiliate), I ran into that problem myself and know a lot of other people who have too. In fact I know people who
were going to an NRA-sanctioned event and because the form didn't clearly say that, their applications were denied.
The best way to do it is to get a hunting licence issued by any State and use that to support the Form 6NIA application. For some reason a lot of people get them from Alaska but I think the Utah hunter training one ($10 pa) is a better idea personally (I have it in writing from ATF they will accept it).
However, CBP might want to see a match entry form to support your statement that you are visiting the place you say you are going to in the US, because your licence might be from Alaska and you are going to a match in Florida for example.
Best idea is to have both, imo.
The green CBSA export declaration card is a good idea too, especially if you're coming back through a major POE like Pearson, trust me sooner or later you will get a jobsworth at secondary.
The other advantage of having a hunting licence is that you can then legally handle firearms in the US generally speaking (and purchase ammunition), e.g. if you go to a rental range you're supposed to have one, some ranges check, some don't, but legally you must have one.
My main bit of advice when travelling with firearms is not to take connecting flights if at all possible, go direct.
Also the mag ban is a PITA because where you're going generally won't have one.