Posts and wire for a quick temp line, as long as you can pound the posts in. Lots of rock around here though.
We have a portable wood frame that is used on the ranges where posts don't work. 4' 2x4 for a base, 4' 1x3 for an upright and a shorter 2' to for an angle brace/support. The 1x3 is hinged to the 2x4 and the brace gets installed with a couple sheetrock screws. A cordless screwgun assembles this pretty quick.
(2) 8' 1x3's for connecting the uprights and we add another 8' section to one end to have a 16' long target line. we usually keep it to 16' max as two people can move it, say for KD on Sunday. If more space is needed we have another set up like the one mentioned. a couple pics from a Colebrook event here:
https://appleseedinfo.org/smf/index.php?topic=53910.0 We usually have 4 pre cut backers , with a little space between them, for each 8' bay. 8 targets max per 16' frame assy. It seems to help in target ID for the shooters, leaves a little room for wind to blow thru also. Backers can be attached with screws to the wood frame or zip ties to either style setup. Space the frames to cover the line width as best you can.
And speaking of wind, it can wreak havoc on either these or a post and wire setup. We've piled firewood and or stones on the 2x4 bases to help hold things down in the wind. With posts use them closer than 10', say 6 to 8' with no more than 3 backers per section. And you'll sometimes need extra posts to brace what you have up. A lot of space between backers on the post TL will help in the wind.
Room for 15 shooters is covered by 6 uprights and eight 8' 1x3's. It fits into the back of a Saturn Ion with the back seat folded down, All the other stuff eats up most of the rest of the interior so it's no extra occupants when loaded.
HTH.