How much of a handyman are you? buy a grind to fit recoil pad and a roll of masking tape. Measure your LOP, and then measure the LOP of you model 500. Remove the stock from the shotgun. Cover the area of the stock you will remove with the masking tape (making sure to cover the area you will be ctting), measure and mark the line you will cut along, mark it, and cut halfway through the stock with a fine saw ( I prefer a hacksaw with a new blade for this.) on the left or right side. Make sure your cut is straight. Repeat the cut on the other side of the stock until you are just shy of half way. Snap the piece to be removed off, and smooth out the new pad mounting surface with a sander. Carefully. Mount up your new pad, mark around the base of the buttstock with a sharpie, remove the pad, and using a bench grinder in small steps grind it until it fits the buttstock. After it fits and is reinstalled, remove the masking tape.
Go slow, and take your time. It is not a hard job, it just requires a lot of attention to detail. If you have to guess on the length, leave it a little long. it is far easier to remove excess wood than it is to put it back on.