Battery Tender, but bafllingly, it failed to work. My theory is that it stops charging at 13V and that is not enough to engage the starter relay. I confirmed this by charging the battery with a regular battery charger instead of the Battery Tender and had no problem starting.
Then I read about Shorai batteries. These are extreme-rare lithium iron prismatic cell batteries (chemistry LiFePO4). These batteries are incredibly small and light (1.6 pounds) and claim to keep their charge for over a year. They come with a bunch of self adhesive rubber pads so you can fit it snugly in the battery box (in the right in the picture, where I also compare it with the stock Enfield battery). They are not cheap (MSRP $153). But it seems to be working as a charm. It was also nice to see Royal Enfield in the pull down menu of their website as one of the options!