I don't know about your 515 but I've had and used a 501 sickle bar mower for about 20 years.
I back the tractor with mower attached and bar in the upright position into the shed when I want to dismount the mower. I have a cinder block and large piece of 4 by 4 lumber on the cinder block. The large part of frame is lowered onto it. I also have a cinder block and 2 by 4 that corresponds with where the end of the sickle bar is. Lowering the mower also lowers the end of the bar onto this block. I also have a scissors jack that is located below the break away arm.
I lower the mower onto the blocks, remove the two mounting pins, and the hitch pin that is holding the top link to the tractor. The mower just sits on the blocks and is stable and stationary until I want to remount it to the tractor.
Works well for me. I hope you'll be able to come up with a good arrangement for your 515. It takes me about 5 or 10 minutes to mount the mower and about 10 or 15 minutes to dismount it.
Tom in TN