I've done it several ways. I've used jack stands under the frame (but in your case that won't work), I've also used a chain hoist in the celing or tie strap from a rafter, but I think in your case I would use my other method balast. Just put something fairly heavy on the seat as far back as it can go, it will keep the front safely off the ground. Last time I did that I used a short heavy punching bag worked great.
Travis