I don't. But don't over think this. Simple four posts cut at any height you want and the 4 wheels attached to the bottom. Two lag bolts in to the bottom of post and two in to the studs that mate to the post. Measure the width and length you want and cut the 2x4s to match, attach to the four posts. Try to keep the contact points on top of the posts. Keep the 2x4 on edge for more strength. Done deal. Probably around $50 depending on what you have laying around. This is really really simple and works great.
