Parts for copper thermowell?

Ok, so I've done some searching but haven't been able to find what I'm looking for. I'm making copper thermowells out of 1/4" OD soft copper from Lowes. Inside will be a DSB1820.. I'm having problems finding some sort of compression fitting to fit this size. These are for the HLT and BK.

As an alternative I might be able to step up the 1/4" to 3/8" OD tubing, but need to make sure everything will fit. I guess I'd prefer not to use the 3/8" as the thermowell because it may retain too much heat, and the temps would adjust a lot slower... not sure though.

Ideally I'd like to find the parts locally (Depot, Lowes, Ace, etc). Any thoughts?

If you have an OSH anywhere near you or an ACE Hardware, they tend to have a better selection of brass adapters. They should have a 1/4" OD compression X 1/2" MPT fitting in brass there. You will have to drill the fitting through. They are made so the compression tube stops and you have to drill out the step.

I'm surprised Home Depot didn't have them in their little bagged fitting area. What you'll probably have to order from McMaster are the silicone o-rings to seal it.

I've been searching for a source on the SS ones that's cheaper than McMaster but haven't found anything significant yet.