I believe a SkiErg is self-supporting. it has a large base. I know its possible because I’ve seen it, but I cannot explain exactly how you position the skierg next to a treadmill.
as for alternatives: here is what I did. about $100 worth of variable resistance pulleys and rope. the pulleys flop around quite a bit the way they are positioned. I have however devised a new setup that moves the pulleys too the floor and provides support for them, I just never made the change. so yes this is a low dollar alternative and it works, just don’t expect the smooth operation found with a Skierg.
since these pics were taken, I added proper ski pole straps to the rope