The best way to do it is check with DoC to make sure the opening of your net is within the regulations. Secondly most guys use a commercially available Ulstron mesh sock, or bag, for their drag net. So if you get the bag first you can make a frame to fit the opening. Get the biggest bag you can. Put some lead sinkers in the bottom tube to keep it on the bottom.
At the mouth of the Waimakariri River the beach is sandy and shallow. Therefore a net which is wide but not that high is better suited. In a different spot where the water is deeper a different shape might be better.
The pipe frame nets are good in the wind as there isn’t much sticking out of the water. However fishing a drag net can be tiring as you have to pull it along all the time – or at least hold the rope. I think one of the big advantages with this type of drag net, particularly at Canterbury’s Waimakariri River mouth, is that when there are many whitebait set-netters in the narrow part of the mouth, there will always be plenty of space down in the surf zone, and so arguments are avoided. No need to get there first and stake a spot. You also get first go at the little fish which must get past the drag nets first even though the river is much wider. When the bait are running you can get plenty with this method.
Generally the best time to fish a river mouth for whitebait is on the first push of the incoming tide. You often find West Coast rivers like the Hokitika deserted on the out-going tide. Then just as the tide is turning the locals appear from everywhere!