Installing downtube shifters on a bike with no bosses or cable stops
For downtube shifters, I prefer to run as little housing as possible.  Your best bet is to look for some new old stock (NOS) vintage parts.  You need two bits: a chainstay-clamping housing stop for the short housing loop to the rear derailleur, and a downtube-clamping cable guide for making the cable bend at the bottom bracket up towards the front derailleur and back towards the rear derailleur along the chainstay.

A quick search of eBay shows both available pretty readily. Several manufacturers make the chainstay clamp, but the Campagnolo 636 is representative.  The best downtube clamp for both a front and rear derailleur looks to be the Campagnolo 626/A double guide, though Huret, Suntour and a few others also made them.