How to minimize hash noise in amplifier mounted near boost regulator
There are two main types of electromagnetic interference.  Conducted emissions and radiated emissions.

CONDUCTED EMISSIONS:

The first thing I would try is to look at the output of the boost regulator on an oscilloscope and see how much ripple voltage there is.

If you see lots of ripple then I would add a PI filter at the output of the regulator using a small inductor and two ceramic caps, and a small damping resistor in series with the inductor, where R>SQRT(4L/C).  The resistor prevents resonance that could boost the noise at the resonance frequency of the filter 1/(2*pi*SQRT(LC)) Hz.

The cutoff frequency 1/(2*piSQRT(LC)) Hz should be several times lower than the 100kHz frequency of your regulator.  Remember that the PI filter is second order, so if you make the cutoff 10x lower (at say 10KHz) you would squash any noise by a factor of 100X.

Also, keep the inductor in your PI filter away from that large toroid or they may couple together which defeats the purpose of the filter.

RADIATED EMISSIONS:

If you think that electromagnetic fields are radiating then steel sheet metal will block both electric and magnetic fields.  It needs to be a magnetic type of steel.  You basically just need to form it in a box around your noise source (in this case the power supply).