It's easy...there are plans on the net...
http://7.62x54r.net/MosinID/MosinRacks.htm I substituted felt for leather [from an old belt using hot glue to hold it in place] for the rifle rest at the top. Really turned out nice....and cost about $15 with the wood, hinges, and locking thingamagig (from Home Depot). I built a couple other styles out of plywood for the basement range. Check it out! You can do it. Get a hole saw for your drill to cut the bottom radius (I used a 2" hole saw I had to install door knobs), a jig saw to cut the opening...a little sanding, some stain and polyeurathane, and vualla perfecto.
SF Tom