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Your main complaint seems to be that the 24 V power rail sags 800 mV when the heater is on.  Three responses pop to mind:<ol>

<li>So what?

<li>It probably isn't anyway.

<li>What did you expect?

</ol>

#1: <b>So what?</b>  I don't see any harm in the 24 V supply actually being 23.2 V when the heater is on.  Unless there is stuff you're not telling us, this supply is only used to power the heater, and to make the 5 V to run the microcontroller from.  Both those purposes should be met fine with some ripple. 

Since you are PWMing the heater, probably inside some feedback loop, the slightly lower input voltage will cause a slightly higher PWM duty cycle with no harm.  The only real drawback is that the maximum heating power is reduced to (23.2 / 24.0)<sup>2</sup> = 93% of what it would be if the 24 V supply stayed solidly at 24 V.  Does that really matter?  If it does, you're probably operating too close to the edge anyway.

#2: <b>It probably isn't anyway.</b>  800 mV does sound like a lot for a real commercial regulated power supply, although I didn't dig out the datasheet to check.  However, with 10 A flowing around, there can be ground offsets in various places.  Where exactly did you put the scope probe ground clip to measure this 800 mV dip?  Try putting the scope probe (both the tip and the ground clip) immediately across C1.  I suspect you will see a significantly smaller bounce.  Also put the scope probe across the supply <i>right at the power supply output</i>.  Again, I suspect significantly less than 800 mV change.

#3: <b>What did you expect?</b>  You say the supply is rated for 24 V at 280 W, and you are applying a 2.4 &Omega; load to it.  That will draw 10 A.  It only takes (800 mV)/(10 A) = 80 m&Omega; somewhere to cause 800 mV of drop.  Consider not only the resistance of the wire from the supply to the heater and the FET R<sub>DSON</sub>, but also the resistance of the ground current return path.  The latter is too often overlooked.  Remember that the whatever the FET source is soldered too has to be able to handle the full current too.

You say your main worry is radiation.  That's a valid concern, but I don't like how you're addressing it.  Trying to slow down the gate transitions is usually a bad idea.  As you noticed, that doesn't seem to have slowed down the switching much anyway.  If it had, though, you might have overheated and blown up the FET.

Reduce R4 significantly, or eliminate it altogether.  The on-resistance of the output driver FETs in the microcontroller probably already are a few 10s of Ohms.

The snubber at right might be doing some good, but I'd look at the waveform on the resistor carefully to make sure.

C1 isn't doing anything useful, and I can't even guess what you think the purpose of R5 is.  A big fat cap like C1 can be useful to hold up the supply after a sudden current demand, like when the heater is switched on.  However, if the wires to the supply are thick enough, there shouldn't be much of a voltage drop to counter in the first place.

RF radiation is about high frequencies, which C1 isn't going to help much with.  Put a few small ceramic caps right at the top side of the heater to ground.  Maybe something like a 10 &micro;F, 1 &micro;F, and 100 nF in parallel to keep the impedance low over a range of frequencies.

However, the most noise will come from the FET drain connected to the bottom end of the heater.  This is the node that will see 24 V transitions 60 times per second.  Try a <i>small</i> cap at the FET drain to ground, like 100 pF to 1 nF.  1 nF has an impedance of 160 &Omega; at 1 MHz, and it goes down from there proportionally at higher frequencies.  Keep the cap small to not cause excessive surge current thru the FET whenever it turns on.

Keep the node from FET drain to heater physically as small as possible, and maybe consider shielding it.