Ambidextrous backup downtube shifter for touring? I'm heading out on a long tour soon on a bike with Shimano STI shifters. That choice was intentional as I don't like bar-ends. However, I notice the gear cables run through stops at downtube shifters bosses and I'm curious if there's a type of cheap friction shifter that would mount on those and could be used on either side that I could carry as an emergency replacement in case an STI ever crapped out. 

Here is a picture of the bosses on this bike: 

If someone could point to one or more specific models that would fit this, that would be great. Again, the emphasis is on cheap. If I break an STI, I would only need this to limp a day or two to the next major city (tour is in Canada). So I don't want to drop $100+ on some bespoke hand-milled stainless steel shifter.
We need to see what is under the cable stops. My guess is that is is probably a standard squared off shifter boss, but you never know. If it looks like this



Then any friction downtube shifter should work as a backup. You want to get
one that is not indexed if at all possible since that way you can use it for 
either front or back derailleurs. Bar end shifters use the same mount, so if you have some of those handy you can see if they fit. The cheapest solution is probably to buy a cheap pair of Sunrace Clamp-on stem friction shifters.

While the levers are different, they can be interchanged. You should probably test this in case there are clearance problems and you need to trim the lever 
a bit. 

As to whether this is worth the cost and weight, I'd say it depends on the remoteness of the tour. If you're within a day's ride of a bike shop for the
entire tour, then it's probably not worth it. A credit card makes a handy backup
for a lot of parts.