Ewan,
Even if you make the disc 20mm wide then you only increse the KE to 180j (still not a lot!!)
If you have access to cnc facilities why not have them cut you, as Christian sugested, a rim biased drum
if you could get say 70 - 80% of the mass at the rim and a width of say 75mm your KE rises to around 0.61Kj @10000 RPM BUT you are looking at around 1.9Kg in weight!! The fundermental problem here is that your dia. of 65mm is too small to allow a reasonable rim speed and therefore an acceptable KE, unless you are going to run it at silly RPM!! +/- 17500!!
If you want my advice, I think you need to have a serious think if this really how you want to go on this bot.

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