This got me thinking. I regularly park with about 12-18 inches between me and the car in front of me (or just far enough for someone to squeeze between the two cars). The idea being to hopefully maximize parking space, maybe fit an extra car in there. I always get mad when I see cars parked with like 5 feet between them when I'm parking, because down a single block you're probably losing 3-4 spots.

But is this a dick move? I often have to shimmy out, going back, forward, back forward, back, out. It doesn't bother me as much as parking blocks away, but maybe other people think it's a dick move.

Assuming you don't care about code and it's temporary, I would just get a piece of scrap metal and make one. Use tin snips or something to cut a circle about 10 inches in diameter, put a few legs on it and mount it on top of the pipe. I'd worry about flow bring cut down with two elbows. I mean it'd probably be fine but just in case. If it's a wood furnace I'd also worry about soot build causing a fire.