What brand of electronic metal halide ballast should i use to operate a 70 watt double ended HQI lamp?

You really want to use electronic?
They are very sensitive (temperature, humidity, mains high voltage disturbancies,...) and their losses are not as much smaller then regular series choke (if you buy a "heavy duty" choke, losses would be about the same as the electronic).
If you would have problem with too heavy or unstable top heavy fixture, consider remote ballast box, the fixture would be even lighter or bottom-heavy, so more stable - for chokes you can use long range ignitors, with electronic is this impossible.
I doubt the longer life is realistic, better use CMH...

A magnetic ballast is not an option because i'm trying to build a 70 watt doudle ended HQI flood light and i need it to be lightweight and portable.

This is exactly the application, where I have the experience the heavier ballast in the base is doing only good. The halogen floods (I mean thise with stand) as sold greatly suffer from the poor stability, so outdoors I frequently had problem to find level place to stand them. The heavy ballast (I use PC-supply box for it with PVC "roof", but better would be to use outdoor rated ballast box) help a lot here...
And for transporting: It sits in the trunk of the car anyway and other tools are usually heavier...

But a bit other topic: Be careful on the lamp. I have VERY bad experience with SLI Rx7s 70Watter (not the bad luck for faulty unit, but already 3'rd piece from the 3 I tried up to now is already failing - it still work, but ceramic on the soocket is already broken; all of them were warranty replacements, so I did paid only the first one, if I do not count the travel and replacement work; it is in a stable fixture). SLI use only tiny drop of cement on the end, what break VERY easilly, even alone.
Mechanical shocks such fixture would receive would be killing for it for sure, in the portable I use some Chinese really cheap lamp (I was curious, how it would work) without any problem - it is way mechanically stronger build then the SLI made in Belgium (but have way inferior light - both color quallity and the lumen output). Osram's HCI seems to be well build as well, but I didn't really tried them yet in the shock-exposed portable flood (the Chinese is doinng it's job there, but once it fail, I will use likely the OSRAM one).

I was going to mount the flood light on a 6X6X4" electrical box so stability isn't really a problem.

And what would be inside the box? It look like good enclosure for the ballast...

But only now I noticed, you are from US, where the 120V mains require somewhat bigger and less efficient autotransformer ballast then the simple 230V chokes I'm used to work with (I have ben mistaken by your early presence here, for a working day, I tought you are somewhere from Europe... :-) ).
But still, my first try would most likely be the simple "core-and-coil"...

I have a 1KW halogen floodlight which i upgraded to 125W MV. I used the enclosure of a burnt computer power supply as box for the ballast (a simple inductor + PFC capacitor in my case)

The floodlight was quite lightweight and installed on a pole together with other stage lights (the floodlight contained 125W blacklight MV) and the ballast box was laying on the earth under the pole

Okay Then, What brand of core and coil ballast should i use?

Well then, I shall use the venture M98 ballast.

What brand of electronic metal halide ballast should i use to operate a 70 watt double ended HQI lamp?

M98? You are from the US and you are asking about it from people who are from Europe using 230v..I think people here should check their background first before posting questions to avoid confusion and accidents.

Quote from: Metalhalide96 on July 28, 2011, 10:11:16 PM

What brand of electronic metal halide ballast should i use to operate a 70 watt double ended HQI lamp?

M98? You are from the US and you are asking about it from people who are from Europe using 230v..I think people here should check their background first before posting questions to avoid confusion and accidents.

Well, I notice this issue and I better check the people's profile. This was a good lesson for me to do it really rigorously, to not relay only on the time in the day...
But if someone want so much "protect his privacy" and do not public his location, I have to only guess and then such guess may be wrong.
So good reason to public it there (at least the region - US, CA, EU, 120V, 230V,...) - it could may avoid many confusions...

HF ballasts can be quite expensive. Osram RX7s are well made but the cement can still break easily if too much p
pressure is put on them.

HF ballasts can be quite expensive.

I don't know, if HF ballast would be good choice, it is asking for resonances in the arctube (these are never offered by brand makers)...
If electronic, it should be the low frequency square-wave AC type.

Osram RX7s are well made but the cement can still break easily if too much p
pressure is put on them.

Yes, excessive force break everything, but that is not the case in the fixture I have problems with SLI lamps - the force was even lower then use to be on halogens...