A great cold frame we made was from two secondary double glazing units my aunt was swapping for real DG. We made a double cold frame to fit it, with wooden pegs at the top that the units could be slotted in to, then held open with bricks or rotating wooden stick supports, or the lights could be removed when things grew big or the day was hot. After many years the glass broke after a particularly heavy snowfall - but it was good while it lasted.
We have used Geoff Hamilton's blue water pipe tube mini tunnels for years. The ends of the tubes are pushed onto short lengths of cane, rather than straight into the ground. To hold the cover in place, roll the edges around canes, then pin them down with tent pegs made of heavy weight fence wire. For weeding (not my strong point) one side is rolled up to the centre top, then clothes pegs used to clip them to the string which marks the centre top. Once you've done one side, replace that cover and slide up the other one. The only bit we are still not totally happy with is how the ends are secured. We pull them tight, tie with string, then hold them in place with bricks - not perfect. The great thing about using blue water pipe is that you can cut the hoops as long or as short as you need, as long as you can get a cover to fit. Or you could use lengths of cover side by side and peg them together to get the right fit.