Eagletac D25C is fairly narrow of high quality. It’s not single-mode though.
There’s also the Eagletac D25C2 which is a 2xCR123 version.
Since you’re looking for narrow these are worth checking out. Both lights are extremely narrow. The head is the same width as the body tube, and both are too narrow for 18350 or 18650.
Plus, the L2M has a 2-piece tube, so you can use an 18350 with the shorty tube, or 18650 with shorty tube + extension.
Roll yer own drop-in, and pot the electronix. You’ll never be able to mod it (the drop-in) once it’s pretty much one big homogeneous unit, but it’ll be as reliable as all Hell.
Use your choice of LED, your choice of 1-mode driver (you or someone said you wanted 1 mode, forgot who), and pot it up good.
I have an Olight T20 Digital XPG2 2*CR123 light that cannot fit a 18650. Too narrow. Tail cap forward clicky switch , modes are changes by twisting the head .
If you need something a bit more rugged/reliable, a Streamlight ProTac 1L might work.
It’s a 1xCR123 @ 275 lumens and can be programmed for single mode only. You can find them for under $40.
They have a newer 2-cell version (ProTac 2L-X) that gives you 500 lumens I think, and they can be had for +/- $10 more.
Same single-mode capability, and can be run on 2xCR123 or 1x18650.
Or, for a bit more (under $80) their ProTac HL-X give you 1000 lumens, but I think the head is slightly bigger than the others, so that might be a deal-breaker.
I have Foursevens Mini MKII and it’s programmable.
You can select to have only one mode if you desired.
Single-mode cr123a flashlight … very rare in China，even two modes 。。。 only i can found is surefire 。。 but too expensive