PRIMOS monopod trigger stick. Also Cabela's sells a hard plastic base that attaches on the bottom with a set screw. The base sits nicely on the ladder stand platform which allows you to place your feet on top of the base for stability.
Two already good suggestions. Tru-Glo also makes the Hip Shot whch isn't too bad either. I use a device--forget who made them, but it doesn't matter as they are no longer made. The Primos momo-pod, or Tur-Glo are about the only two I have looked at recently. The problem with the Primos sticks, and I have personally witnessed, and experienced this--the bottom section creeps/leaks down. Also if you ever get water from rain inside the stick--it can/will happen--it really causes them to leak down, and the stick becomes useless until you get things apart, and dried out. For this reason I have not purchased a newer generation 3 stick, becasue they have the same problem. Read the reviews on them--they are mixed. Over all--if you take care of the stick it should work fairly well, but durability--meh. They aren't cheap, but are made cheap--in my opinion.
Dad and I both use a Primos Trigger Stick.....they do have some issues as stated, but overall they serve us pretty well for use with crossbows from blinds or tree stands (and shotguns from the blind during spring turkey season).
Is there an option to add a shooting rail to your existing stand? I know there are strap on tyoe rails that can be purchased. If my memory serves me right I think it was either Cabelas, or Bass Pro that sold them, but I haven't checked. They attached using ratchet straps, and a buddy of mine has used them in the past.