Mike is right in that the front lowers need to exhibit good design.
I tacked up a couple of preliminary iterations before I settled on the final configuration. Given that I was using light weight tubing I decided to triangulate them in a vertical plane to prevent flexing due to spring loads. The ERW tube I used is just your normal exhaust type tube. Cheap as chips, easy to work and more than adequate for the job.
With 3mm tube, if you feel it needs strengthening, simple strategically placed gussets should suffice.
After you have built them, if you have any concerns, for peace of mind just keep an eye out around the lower shock mount area for signs of cracking. You should see warning signs long before total failure provided you only have one thin spray misted paint coating. Powder coat, chrome or HPC type coatings are a no-no.