I've only had one convenience store manager ask me to leave my bike outside. Other than that, the small grocery and c-stores have never had a problem with me walking in with my bike. When I do lock it outside, I look for areas clearly visible from the window, lots of passersby, and/or security camera coverage. I like to use restaurants with walk-up windows or table service and outdoor seating. One of my favorites doesn't allow me to bring my bike into the seating area but I can chain it to the railing near an empty table and take a seat literally an arm's length away while food and drink are brought to me. The bike is out of sight and reach for only about a minute while I walk in through the front door.

BTW, small towns being safer is a myth. I know of several people who have lost their bikes or had attempted thefts in rural communities. I caught an individual trying to get a bike off of my hitch rack at a gas station in a town with a population of about 1,200. He had the two straps undone and had lifted the bike out of the cradles. If it hadn't been for the cable lock to the truck frame, he'd have been gone with the bike. Stupid thing was that it wasn't even my bike, it was a donation I was taking to a bike co-op in a larger town about 15 miles away, where the would-be thief could have earned a bike legitimately with a couple evenings' work (the co-op asks for 5 volunteer hours to earn a bike and helps the patron put it back into good working order, usually taking about another evening or two).

Perhaps a small u lock, just big enough to fit through both cainstays, or top tube and a pole. the idea is not to leave enough space for a bottle jack to be inserted. Also a cable about 6 feet long is enough to run through one seat rail like a dog collar then both wheels and the loose end into the u lock.

You don't need to run a U-lock around a maintube or stay, just lock the portion of the back wheel that is between the stays to a suitable post, rack, bench, etc. Since the wheel won't fit through the stays, the only way to steal the frame or rear wheel is to cut the tire and rim or a stay (not worth it for most thieves or most bikes). This allows you to use a lock with a shorter shackle which makes it harder for someone to use a jack or other device to pry the lock open.You still need some kind of cable for the front wheel and some way to secure the seat post.   Nothing reasonable will stop someone with a good cutoff wheel on a portable grinder, if they are that serious all you can do is chain your bike to a 6' 4" State Trooper.