Just got a cover bet feeding ring and fitted it to my D&D net cover. Simple to fit if a little fiddly with 8 very small nuts and bolts. You fit it then cut the net through the ring. Nice neat job and good value for £11 delivered.
A couple of years ago I made a small feeding ring out of 2 of those rubber rings you get on the cartridges of RO filters. I glued them back to back on the net then cut the middle out. I then used a bottle top to put over the hole so no fish would jump through the hole.