We just built our this year for 40 foot hunter, we did not incorporate the back stays though, we figured Hunter had a better idea of where to put them than we did... We used 1and7/8 stainless OD Tubing, from a salvage yard,, we put the main bends in with a tubing bender, and, NO, we did not have to fill it full of sand to prevent it from wrinkling...I tried a sample piece with an Enerpac Tubing bender and it worked great...
Fortunately we had our brother, being a boiler maker, do all the welding, using a 120 v portable welder, that did a flawless job on stainless... We will be incorporating the Bimini later next year, we also added 3 " pipe for the Dinghy Davits as well, works awesome !!!!!