Well, tripod at one end, 1200L at the other end standing on its lens-hood, I reckon you could get a nice 2-person sized tent out of that...


Meanwhile, there's also the Pentax Takumar 6x7 300mm f/4 lens, I got one off ebay for $60 or something, IQ isn't so bad, even wide open in a very quick test it's not too dissimilar to my 70-300L. Add a pentax 67 or Canon-mount 2x t/c and a 67-ef adapter (with tripod mount), and you're up at $150 maybe.

Also I've got a Zeiss MC Sonnar 300mm f/4.0, again about as good IQ as the 70-300L (probably better than the Takumar because it's Multicoated), but it cost me about $300, the soviet teleconverters for pentacon six are very nice and cost maybe $40... (i might try taking a shot with this combo on the weekend)