I have a 50 foot slackline setup from Slackline Express but when we moved to our house the tree situation was unfavorable for slacking. As I result, I have experimented with several A-frame setups. My first setup was a single A-frame consisting of a sheet of 3/4 inch plywood cut into the appropriate shape. This worked for a spell but ultimately started to break after some jumping and bouncing on the line. Following the various examples from the web I built two A-frames from 2 x 4 and 3/4 inch plywood. I am quite partial to the Kreg Jig pocket joinery setup as this makes aligning and joining the bits of wood easy.
Regarding anchor creation in the absence of trees: Most sources seem to suggest pounding three stakes into the ground and creating an equalized anchor. For the sand and clay soil in my yard I found that a single auger anchor on each end works fine. Get the longest and widest one you can find, mine were about a meter long and the auger was about 15cm in diameter. The augers shift a bit when they are first loaded but shortly lock firmly in place. As usual check that there are no utilities where you are planning to place the anchors. This is ideal for a permanent or semi-permanent setup since the augers get bent when they are loaded and are a little tricky to remove. You may be able to get around this by screwing the anchors in at an angle but I have not tried this.
