Decided on this

[rp3X]

Dont concentrate on the square plate as it could have been welded off square,instead concentrate on the upright part of the "L" and you wont go wrong.

So may I ask cos im a bit newbie how would one make sure its upright? Do I sorta use the spirit leveler on all sides and top of the end of the L to make sure its all in level and as straight as possible ?

 

[Rumpelstiltskin.]

Yes plumb all round mate.