MSA ARC FIA and just about every motorsport regs say "One continuous loop withouit joins or joints", so in box that could be intreresting
Frankly I wouldn't bother, it still a shed load of work, and will not be up to it, why not save the time your going to spend and just buy something ptreformed ?
This way if you have a biggie (and don't think that just cos you only do Play days you'll not have to worry

) you stand a chance of surviving,
.......my last comment is made on this basis ....and not as some smart arse comment ....
If you make it out of box, and the welding and joints are not absolutely 100% 1st rate, then apart from the real prob of injury from a roll, this box cage could easily come apart and a lump of sharp broken off box section then looks for a new home ?
Serioulsy do not do it
Nige