Jim,
I am using thin teflon washers for spacers. I have found that the spacing along the line and number of them doesn't seem to make much difference. In the straight sections they are as much as 5" apart while curves are as close as .8". There isn't much difference in temperature between the straights and curves. That is just feeling the line, not using instrumentation. I use round washers. The 3 and 4 sided ones are just more trouble to make and don't help much because they are only touching the outside line in one place anyway. The washers are a press fit on the inner line. When I started, I had a series of failures until I realized that when I pumped a line down or let air back in, I was blowing the washers out of place. One of the biggest factors is leaving space for the inner line to shrink. I calculated that the last line I made would shrink .260" from room temperature to LH temp in the longest straight section. I use smaller OD spacers in corners to allow some movement and push the line in to make the spacers in curves go against the outer wall, giving them max room to pull in.
In my view, doing this without a mass spec leak detector is pointless. I use an old VEECO MS-9, which is ancient technology, but a leak is a leak and anything that will detect it is doing the job. Mine has a Stirling engine chiller on the vapor trap, so all I need to run it is electricity and compressed air. Since it can run unattended, if there isn't a leak, I leave it on to pump the lines down. SS makes a good heating element, so I clip jumper cables to each end and put enough current through it to heat it to about 100 C, leaving it overnight. In the morning, the ion gauge on the MS-9 is down around -6. Pinching off the copper evacuation tube was a whole R&D project of its own. Simple, once you figure out how, but frustrating until you get there.
We experimented with self cryopumping and found that it worked, sort of, but was nowhere near as good as a serious vacuum. Doing it with LN is interesting because the residual air only condenses into a liquid, not frost. Apparently it drips on the outer tube and boils, repeating until it all gets somewhere that the drop hangs on the inner line and stops dripping. LH works better because the air condenses as frost and stays in place.
These are used in slam freezers that freeze tissue samples so fast that they skip the crystalline phase. I tried to promote a project for a replacement like you use, but even with the rising shortage of helium, I couldn't get any interest.
Bill