What varieties of tomatoes did you have success with? Did you find they stopped fruiting in the heat? What did you use to cover them?

I'm in a hot and humid area -- but not as hot and humid as Malaysia -- and any tips are very useful!

All my tomatoes are grown in pots. I do not trust the soil around here.
The successful ones so far are;
Mexican midget, Marzano, Yellow pear, Rutgers red.
These all produced reasonable amount of ripe fruit.
Now trying Sioux, Legend and Burbank slicing-they are flowering now and look healthy.
There are 14 other varieties that are designed for heat, but as you say, it flowers and dies off.
To cover them, I built a basic frame and placed clear corrugated roof sheeting over them. Then under the sheeting, I lined it with shade cloth. It gets to 46deg in the direct sunlight.
Water twice a day and they are happy.
Hope that helps.

I live in Malaysia. Penang to be precise. I have had exuberant and heavy fruiting with tomatoes in my hydroponic system. The plants weren't hybrids, just grown from seeds obtained form supermarket tomatoes. So, I believe the weather in Malaysia is not the limiting factor at all. I have since moved over to organic gardening after developing a phobia for the possibly high nitrite/nitrate content of hydroponic produce.
I have just prepared my tomato bed, digging/loosening the soil about a foot deep and adding topsoil, home-made compost,dolomite,MgSO4,Rock Phosphate,fish meal,soya meal,legume green manure crop. (can anyone suggest anything more?). pH 6.8. It has been sitting there for about 6 weeks now while the tomato plants are growing in their grow-bags. I have been fertilizing them with fish emulsion (Oh! the pong). Incidentally I have sent a sample of the fish emulsion for culture to exclude E.coli and other pathogens, as it smells exactly like sewerage. Hope to transplant this weekend. Planning to plant them deep and at an angle of about 30 degrees as I understand that it will help the entire submerged stem to send out roots giving the plant more vigour. I have seen my hydroponic tomatoes grow tons of roots and the fruit production appeared to be proportional to the root system. Just an experiment. I will only post a picture if I get heavy fruiting not otherwise!

Any pictures of your seedlings?

pH of 6.8 is almost perfect for all garden vegetables, including tomatoes.
Back in USA we, add compost, composted manures(horse, cow..) to condition the garden soil, as well as fertilizes it.

Take it easy with fertilizing. Read more about it. You want to get tomatoes not a lot of lush green top.

good luck !