Joe,
My most recent similar bend was on some hardware store ¼” aluminum tube used for the pilot light on a furnace. I’d have to go measure my sample, but I hand formed it around a grooved wooden disk to a centerline radius of about 3D. (Going by a photo I took.) It was not particularly difficult, but going tighter would take a lot more effort. Using 3D applied to your 5/16 tube, you are looking at nearly an inch of bend radius, which is twice what you hope to achieve. So this is going to be challenging. I think I have a book that has aerospace minimum bends in it, but that will take some digging.
Using Cerrobend material is an industry method of doing what you are trying. As such, my thought is that it would be more suitable than pitch, sand, frozen water, or weed whacker string. It is a very low melting point alloy designed for just this purpose. The lowest melting point alloys contain cadmium, so know what you are doing before you pick that. The lowest Cad-free alloy melts at 203F, which isn’t up to the full annealing temp of aluminum. So you don’t have to risk working with heated cadmium for this method. Here's some info:
https://www.industrialmetalsupply.com/Products/specialty-metals/cerro-bend#1You can also look in McMaster-Carr. They try not to use brand names, but if you search using Cerrobend, it will take you to the same stuff.
You could also try solder, but the melting point moves up into the mid-400F range. I’m sure you have some of this around, so you could try that almost for free. Because of your very tight desired bend radius I would combining the solder plug with a grooved wooden form. You can make a straight grooved wooden block to use as a wiper die. If you get serious, make a steel strap that pivots around your radius block and holds the wiper die in tight. You are now getting close to….
The best way to bend tubing is with an actual mandrel style tubing bender. These have a series of hardened steel balls all linked together that fit into the tube. (Looks kind of like a keychain....) The machine pulls on the tube, so you do not get failure on the inside of the bend from buckling. The smallest one of these I have personally used was either 3/8" or 1/2" diameter. It has grooved guides somewhat similar to the DuBro tool, as well as clamps. Go to Utube and find some videos on mandrel style bending. It may give you some ideas.
If you are using K&S tubing, I'd be concerned that the .014 stuff is so thin that it will fail in a tight bend. I’d try either their .049 or .035 wall tube. I recently wanted to bend some 3/32"(?) K&S aluminum tube. I tried Larry's weed whacker trick but it did not work well for me. The tube buckled before I got the bend I needed. I will try some of these other methods when I go back to that part of the project. (It was for a cockpit control stick.)
Here’s a possible alternative, if you are going to paint them. Make a curved mandrel out of something suitable. (I’d try a piece of fuel tubing unless you have a piece of Teflon tubing handy.) Wrap it with bare magnet wire using a fairly heavy gauge. Lay the wraps down perfectly with each against the previous. Silver solder over the whole shaped coil. Extract mandrel. File, fill and paint. Windy U. sold molded plastic exhausts for his Spitfire project. No tubing involved. Less fidelity, but less headache….
Best of luck. Show us how it turns out!
Dave
Looks like Sparky beat me to the send button. Where there's Sparks, there's Lightnin'