Reddit mentions of Bar Mitts Cold Weather Road Bicycle Handlebar Mittens fits Shimano Shifters with Externally Routed Shift Cables, Black, Large

You have to buy special bar mitts, not the same ones designed for flat bars. But you can get them, even with routing holes for external brake cables. I have a pair of these and I'm extremely pleased with them. They're quite thick and very warm. In fact, if it gets much above freezing I have to take my gloves off completely as my hands get too sweaty. But I rode them in 15 degree Fahrenheit weather this morning with thin gloves on and my hands weren't cold at all.

These are bar mits and have become insanely popular where I live. Lots of fat bikers use them. Bunch of people I work with use them as well. Work with flats and drops.

http://www.amazon.com/gp/aw/d/B002VKABAG/ref=mp_s_a_1_1?qid=1415883930&sr=8-1&pi=SX200_QL40

Well, you are riding with drop bars whereas I ride with bullhorns, so the design would be different. But I don't think it would be that hard to mimic this Bar Mitts design. Assuming you have some basic sewing skills, you could make cut two pieces in the basic shape, sew them together and then use velcro instead of zipper if you wanted (this is based on your bike photo and assuming it has "external cables"). Of course, you'll want to do a solid job on the sewing so air doesn't cut through.

However, it may or may not save you money. I bought a bunch of vinyl on sale of multiple projects, and hurriedly threw my pogies together. I made my own mostly because I didn't find bullhorn styles out there and wanted a bit of DIY.

I do see that Amazon has the Bar Mitts on sale for under $50 for most sizes/colors.

Edit: I have been linking/referring to Bar Mitts. I do not work for them nor have I used them. So take that for what it's worth.