A disc ceramic that's big enough will work fine. That's what Henry was shipping for one customer's amplifier when he called them about it. A big, fat one mind you.
The doorknob is better because it has more metal in it. More metal means lower resistance. Lower resistance means less heat from the current passing through it.
First one problem. Finding a current rating for a ceramic capacitor can be tricky. Figuring out how high the RF circuit current will be is another problem.
The 'more metal' problem can be solved by a higher voltage rating. Not because of the circuit voltages, but because of the capacitor's construction.
The higher voltage rating means you have to put more distance between the two electrodes. But when you do that, you have to make the electrodes larger to obtain the capacitance you need. Larger means more metal.
Using a capacitor rated at 15,000 Volts in a circuit that won't put more than 5000 Volts across it gets you that current rating, and prevents the capacitor from overheating and failing.
Metal is metal. A ginormous disc-style cap that's nearly a quarter inch thick will typically work as well as a doorknob type at the same circuit current.
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