Base - slightly out of square

[big-all]

I built this base and measured everything diligently as I went along.

View attachment 423492

When I measure from corner to corner, there is a 1cm difference. I’ve tried kicking it in the corner but that knocks it out on the other end.

Is there a better way to do this?

5mm out off square is zero in reallity[+5 and-5 as you average out]
it may be 100% square but a small dip or rize mid point corner to corner can also cause a wrong reading

 

[Jupiter01]

UPDATE - following your advice, I did a couple of things today. Temporarily screwed on the OSB boards and pulled them on slightly to square each section up. I also put this string line and checked along the length. All looking good. Thanks again