Search CPFMP for Fivemega 18mm bodies sold by WonderLite... You can PM him and see if he has any in 2x18650 config... But I know he has some extenders as well so you could go with 1x18650 host in combo with 1x18650 extender... Fivemega are generally very good quality hosts...
Second option for you is to try to find Surefire 9P with A19 extender then bore both for 2x18650 (like yifu suggested)...
Then you have an cheaper option of getting Solarforce L2P host and get their 18650 extender... this is the cheapest route, Solarforce has a nice quality hosts as well, not as tough as first two options but durable and, like I said, nice... and for a fragment of price compared to FiveMega and Surefire route... these hosts come with inner diameter of 18,6xmm already so no boring required... on a side note, I find L2P tailcap funny but you could order S10 to go well with that long combo you have in mind...