There is a lid, not a cap, made specifically for this application. It is dark green plastic. It has a lip that sits flush with the rim of the pipe. The lip has a foam gasket (for sewer gas sealing), and is predrilled for stainless steel hex head screws that come with it. The piece is durable enough for foot traffic. I get mine at a local pipe/ underground utility supplier. Unfortunately, I haven't got a clue who the manufacturer is. It's early AM here. When I get out to one of my jobs, I'll see if the manufacturer's name is on the piece somewhere. They run about $35 in that size.
EDIT: Found it on the Google. septicsolutions.com tuf-tite brand heavy duty riser lid, 24" Don't know the specifics of your issue, but here we would end up with the lid buried with six inches of dirt cover. It keeps the sewer gasses under control and it isn't too deep to dig out easily, when it's time to pump out. Make sure the lid and the riser to tank connection are both water tight. Ground water leaking into the system is not desirable. Have fun.......