The grey cover is Home Depot Eco Foam I had lying around, the target box is filled with about 80lbs of cotton rags. I cut a cover to go over the top hole and caulked it in place.
The blue tarp is bungied over the box when I'm not using it. We've had monsoon rains and hail, it's bone dry. All of the wood is waterproofed with 2 coats of Minwax "Helmsman".
The wheels are 8" solid rubber, which work OK in the grass but I couldn't find any air-filled knobby tired ones, which would be better (fortunately I don't move it much). The two in the back are fixed, the two in the front rotate and have locking mechanisms on them.
My arrows penetrate about 6-7" through the box and stop fine. Simple to remove. I didn't put a compression bar because I didn't think I'd need one with all that cotton, and so far I don't. I've probably put about 300 arrows into it so far.
The attached plans are for Google Sketchup, which is a free program you can download.