What sort of crossplank and springboard have you come up with?
Although most resources will suggest "chocks", a boat will usually be able to keep it's runners on the ice, in most conditions, without them. The chocks are the metal mounts which runners are bolted into, so that the runner can "rock" a bit. But crossplanks have a bit of "twist", and ice is usually pretty flat, so runners bolted right to the springboard work pretty well.
That being said, runners made from 1/4 inch thick angle iron work very well, and are cheap and easy to make. For a boat of your size, a runner length of 18" to 24" would do it... Sign post stock, galvanized, can be gotten in 2 1/2" x 3 1/2" size. The 3 1/2" dimension should be used as the vertical.
Give the forward edge a curve, so it "rides" over bumps. Sharpen the bottom with a file or grinder, finishing up with a file. An angle of between 45 and 55 degrees is good... 45 is better as an "all around" angle... as in both soft and hard ice.
It is essential to align the two rear runners perfectly parallel to each other, by the way, so they do not "fight" each other as you move forward.
Of course building a go fast racer is a different story... here you would have alloy chocks, or pillow blocks with bearings, and composite runners. But for just messing around, simple runners as described work surprisingly well... with such runners on a windsurfer rigged "A frame" boat, I was timed at just over 60 MPH.
You could also look around for a used set of DN iceboat runners and chocks... try eBay, or put "used runners DN iceboat", and like that, in the
http://www.google.com search engine. Rich.