What I have are some pieces of oak with channels routered in to fit over the sills. The trolley then goes slightly behind the proper sill jacking point leaving room to put the axle stand at the proper point, both with the oak to protect the sill. This is surprisingly stable, especially as the axle stand makes an indentation in the oak. I would rather do this than use the main jacking points, which don't seem to be very secure. If I do use the main jacking points I put some tough rubber between the stand and the jack point, to at least provide a little grip. The only problem i have is getting the jack under, I have to drive onto a piece of wood to get enough clearance.