Well the originals would not have been stainless but simple mild steel CHS, (circular hollow section or tube if you like).
The wall thickness is important and this should be 3mm for the general length of these things which is 50-100mm.
The diameter will be governed by the hole it needs to pass through on the chassis, the tube should fit snuggly in this hole.
Length is critical, this needs to be slightly shorter than the distance from one side to the other, to short and your excessively crushing the chassis, to long and your not clamping fully to both sides of the chassis.
Beyond that, you will need spreader plates which are generally heavy duty large diameter repair washers but could be purpose made, this is to cover the larger hole and most importantly spread the load out into the chassis leg.
Bolts if you have them are possibly passed their best, locking washer certainly will.
To my mind then, I'd wait to hear from Towsure in the hope they will supply a fresh kit, without a thorough understanding of what you need to achieve, as only outlined above, you will otherwise be asking for trouble