My U-bolts are 5/16, but they are way overkill. I get all my stuff from Mcmaster-Carr. Easy online ordering, and they'll sell to anyone, unlike some industrial suppliers. Their catalog is simply amazing, if you like industrial stuff. Notice the polished acorn nuts I put on the outside- gotta love the bling!
My drum will hit 450 or so with a fully lit load of coal. My firebox is 16" in diameter. Normally, I fill it about half way, with a few chunks of wood in it, and add about 10 lit coals. I immediately get the food on, and run it with the exhaust wide open, and just the one vent open. It settles in at 225 all day with that set-up.
I suppose in a real frog-strangler, a measurable amount of rain could get in my riser, but I can't imagine it would hurt anything. My riser is electrical conduit for the time being. I'd like to do something in SS eventually, but I'm still playing around with what I want to do.
My U-bolts are at 2" and 8" from the top. I drilled out a bolt to make a holder for a 12" long thermometer, and put it halfway between grates. I made my firebasket so that when it sits on the lower set of U-bolts, it just clears a grate on the upper set. To be honest, we grill more than smoke, so I'm glad I went with that set-up. I made a handle for the firebasket so I can lower it to the bottom for smoking.
I'm not sure if a SS basket would be worth it, unless the price was right. SS only remains rust free up to a certain temperature, and high temperatures seem to reduce the 'stainless' component. After reading about mini-baskets, I'm going to make one for very low temp smokes. I like to do fish, and even 225 is a bit high for fish.
I'm very intelligent, but I mask it by doing stupid stuff.