Big melons

I've been trying to grow melons for the last couple of years in an unheated greenhouse. So far the best size that I've managed to achieve is maybe tennis ball size - and that was a particularly good year in terms of sun and temp. In general they either do nothing at all or get to golf ballsize then give up. I suppose that temperature and light are particularly important - but are they 'do-able' in an unheated greenhouse?
Thnx in advance 

I grow mine in an unheated greenhouse get best results from the cantaloupe types grown in ring culture like tomatoes. They can be rampant so I grow them as a cordon up canes to the middle of the roof pinch out growing tip and all laterals at five leaves. I tie in canes horizontally about a foot apart and tie the laterals to them, and when the side shoots flower pinch these shoots out to two leaves past the flower or they will overrun the place. I only let four of them grow on each plant so you have to be ruthless. To get big melons they need plenty of water and feed I used a growbag last year split up into the pots and fed the fruits weekly with been and queued multipurpose concentrate. I got some melons size of large grapefruit one did two of us and the others were smaller about the same size as normal grapefruit, variety was 'Charentais' deep orange colour and very sweet, this year it is 'Eldorado' from Seeds of Italy they are in the propagator at least one has germinated, they will go out around May – hopefully.

I've had success growing them alongside cucumbers in tomato grow bags, I grew them through the racking frames, removing the shelves to give them a strong support. Don't know if they two plants complement each other or if it was the grow bags. This was in an unheated greenhouse and they got to the size of small footballs. I only allowed 2 or 3 fruit to grow at once though. I pinched off the rest of the flowers and supported the growing melons using a pair of tights as a hammock.

I also kept their feet dry same as the cucumbers

Seems to be the sort of weather for going through my photo collection today!

Attached pic is of Sweetheart melons growing in my greenhouse in 2011 - they are in the planter in the left of the pic on the GH staging, and you will see I finished up deploying quite a lot of assorted sticks, wires and string!

Sweetheart are not a large-fruited variety which may make them a bit less chancy than some others, although I have to confess they conked out last year before even getting to the flowering stage. 

(As this is a GH/poly growing thread I'll move it to the GH/poly board.)