Seat mounting crush tubes

[BenBrooks]

Hello all,

For mounting my seats, I welded in 2 25mmx25mm box sections cross members between the chassis outside and the transmission tunnel for each seat. To fit my seats onto these I believe I need to put in crush tubes?

Can I simply buy some seamless metal tubing and weld it in place for the bolts to go through? If so, what wall thickness should I get? Or do crush tubes need to be a special strength to comply with IVA regs?

Thanks