400w hps safety question

Hi people,
Just after some advise/info, I have 2 600w hps lamps running at the moment and I am wanting to add a 400w I have spare for extra lighting.
I have 2 fat bushy indicas in their 2nd week of flower and they are so thick and bushy light isn't getting much penetration, I have bent the branches around but the foliage just fills up the gaps fast.
I have only ever used the ballasts which the lamp fitting plugs into, so my ballasts sit on a table which is fine but the 400w is the type with the fitting attached to the ballast and I have never used these.
My house is an old leaky farm house so i don't trust the constant weight on the possibly weak roof structure.
So my question is how hot do the 400w ballasts generally get? Will I be safe to sit it on something on the floor between my plants?
I am thinking of putting it between the plants on some polystyrene or a block of hardwood with the bulb pointing upwards. I have my 600w ballasts sitting on a wooden table with no worries but will the extra heat from the bulb being so close to the ballast be dangerous on the floor?
My sister burnt my mothers house down by leaving a hair straightener on a bedside table turned on, was low heat but over 2 days was enough to ignite so I'm a bit paranoid lol.
Thanks for any thoughts

I wouldn't suggest polystyrene, I would use a brick (which I have used before), or maybe something like a breeze block (the lighter aeriated bricks for internal walls).
I have a 400 w ballast and they get as hot as the 600, I hang mine on reasonably thick MDF which I then attach to the wall. I have 3 ballasts attached this way and have never had any problems.

Thanks mate the brick sounds like a winner, its not gonna ignite! I only thought poly because it doesn't burn just melts and doesn't seem to hold heat just insulates. So you're saying you reinforce the spot where you hang them from?

Good quality 400W ballast case temperature is 45 degrees Celsius, the bad ballast's case could be as high as 70 degrees Celsius, very dangerous!

Jack London said:

Good quality 400W ballast case temperature is 45 degrees Celsius, the bad ballast's case could be as high as 70 degrees Celsius, very dangerous!

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Thanks for the info i'll keep it in mind as it is an old well used ballast