I don't know of any any arcade holsters that will fit the act-labs gun, so you're left rolling your own I believe. I've heard of people using various items like cup holders, rubber molding, large pipes, etc. to mount them (whatever works best for you). Some have even made traditional leather holsters as well. Others have simply cut a hole in the front or side of the cabinet and lined the edges with felt. I planning on using a 3" black pipe coupler lined with felt.
Good luck in your quest for the perfect holster! If you find something you think works well, let us all know.