I bought a few feet of that nylon strapping (like a backpack strap) and some of the clips that go with the strap (the clips that adjust and snap together - I hope that makes sense to you!) I strap the legs of the crib around the boards under my bed. I like the clips so I can unsnap it to get under the bed for cleaning. For me, it works better than the bungee cord b/c I can adjust the strap to the exact tightness and I don't have to go way under the bed to hook and unhook for cleaning.
I used foam from a fabric store to wedge against the mattress. Because I have a platform bed with a frame around it (Ikea Malm,) I needed foam between the crib mattress and my mattress to cover the wood frame. I cut the foam using an electric bread knife, then covered it with the same fabric that you use to make cloth diaper covers. (This was a bit of a paint to get right. The biggest problem was the design of my bed. Also, the crib I bought had extra big end posts that were much thicker than the side railings, so that pushed the crib mattress away from my bed even further. I did finally get it right, though.)
I use a mini-crib b/c I don't have the room for a full-sized crib. Really, she sleeps mostly on the foam between the two beds, more than in her own crib or in my bed. I wish I could get a crib that is only half as wide b/c she really doesn't need all of that space. It would be great to just have two feet added onto my bed as a safe little haven for her, rather than a whole crib.
I use a square of rubberized flannel to make a smooth, waterproof surface over where the two beds come together. (Since the foam is between the two beds, I didn't want two little splits right where she sleeps the most. Probably not needed as much if you push the mattresses together and put the foam (or other filler) between the crib mattress and crib railing instead.)
My daughter is still pretty little, so I keep a little basket in her crib with my nighttime stuff (water, snack, nipple cream, burp cloth, etc.) since the side car prevents me from having my nightstand.
We are actually transitioning now to a different bed (for me.) This new bed is much higher than my old Ikea Malm bed, which presents new challenges. I tried to set it up yesterday and I now have to have the crib set on the highest height (was on the lowest) which makes me nervous as my daughter is nearly sitting up. My solution was to put the crib in the corner, then my bed up against it (thereby blocking 3 sides - two against the wall, one against my bed.) I think I'm going to push a chest of drawers up against the last side so there is no way she can climb up and fall over the railing. Other ideas: get rid of the box spring, get a half-box (box spring with half the height of a regular one,) or get rid of the frame and put my bed directly on the floor
Hope this isn't too confusing!