Two good sized dowel rods cut to length of the width of inside the shack. Put them inside the shack on each end. Make a mark on each side where the middle of the holes would be. Then drill a hole through the dowel. Take rope and lay it from dowel to dowel over the holes of the shack. Feed the rope through the dowel holes and knot them off. Then mark on the rope the middle of each hole.
Now before setting up, I can stretch out the dowels getting the ropes tight. Then mark in the snow/ice at each mark on the rope. Drill the holes. Clean up the area. Slide the shack over and you are good to go. I just keep the dowels/rope in the shanty when I pack up.
Solved a hole ton of problems.