18350 Battery Holders...

[Cool_Breeze]

...appear to be in the pipeline.

Keystone Electronics Cylindrical Battery Contacts, Clips, Holders & Springs | Mouser

18350 battery sleds and contacts by Keystone

 

[david4500]

26650 holders also. Both were originally supposed to be delivered to Mouser beginning of February.

 

[Matt Heaton]

Yup here they are
1109 Keystone Electronics | Mouser

 

[jblack741]

Dual 18350 - Dual 18350 Battery Tray | Modz By Nasy

Dual 18650 - Dual 18650 Battery Tray V2 | Modz By Nasy

Dual 26650 - Dual 26650 Battery Tray | Modz By Nasy

I've used the 26650 both dual and single. I like the clearance they have and the quality is outstanding. 3d printed for the 1590g style boxes. Prices are comparable to Mouser/digikey but shipping is cheaper and very fast!