2x4 for stringers isn't the greatest (depends on weight). you might see some lateral movement if there is no bracing other than ground and at the top.
2x4 are fine for the rungs as long as the they are not pulled out from the stringers and screws (if used) don't break. nails plus heavy wire stapled over the rungs and along the entire stringer is cheap and safe.
have built a few in the past (even fold down versions). sorry, don't have plans at hand. but pictures show the idea. regarding measurement: resist to put the rungs too far apart, measure your kitchen chair for the basic geometry of the seat and put the shooting rest on when the stand is in place