OK guys looking at this completely laterally.
Hear me out before ridiculing and apologies for any typos...

This requires an installation of SOX on a personal home computer, NOT, the said machine.

Forget the intended machine at this point. The only assumption to be made is that OP is allowed to transfer a file to said machine.

How about using a home computer of any kind, in this case my MacBook Pro, circa August 2012, OSX 10.7.5, default bash terminal, feeding a white noise generator into the mic input of the 'on_board' sound system using an avalanche diode idea via a zener.

1) The noise is definitely random, white thermal noise in circuits by definition IS.
2) Theoretically any raw data file can be almost limitless is size.
3) Because it is byte sized per sample and raw then you could decide whether your values are 8 to 64, (or more), bits in length.
4) The passband of the computer's mic circuit is peculiar to the machine recorded on. EG 300Hz to 5KHz or much wider, who knows, adding to the randomness.
5) You can't guarantee that these specifications are even flat and my have small peaks and troughs throughout the passband.
6) Aliasing WRT the sampling frequencey is an added bonus to the randomness, assuming linear interpolation.
7) If an added amplifer stage is needed to full scale the mic circuit, then another bonus to the randomness due to its own small internal noise generated.

That is all I can think of at the moment.

Once you have the file, transfer it to the main machine and decide what bit depth is suitable and use any floating point value as your multiplier.

This is what I term as "my weapon of last resort". And no matter how stupid this looks it works.

Code:

#!/bin/sh
# random.sh <seconds[1 to 59]>
SECS="$1"
/full/path/to/sox -q -V0 -d -t raw -r 96000 -b 8 -c 1 -e unsigned-integer -> /tmp/random.dat trim 0 00:"$SECS"
exit 0
#
# Ultra simple noise generator.
#
#                    +-------> +9V
#                    |
#                 ___|___ +
#                   / \  |
#                  /   \  ZD1
#                 '_____'
# Mic I/P    | |     |
#  O----*----| |-----*
#       |    | |     |
#      	/     C1     /
#       \            \ 
#       / R1         / R2
#       \            \ 
#       /            /
#       \            \ 
#       |            |
#  O----*-----*------*-------> 0V
#           __|__
#           /////
# Parts:-
# ZD1 .......... 1W, 6.8V zener.
# C1 ........... 1uF non polarised capacitor.
# R1 ........... 2.7K to 4.7K resistor, any power and tolerance.
# R2 ........... 82 to 120 ohms resistor, any power and tolerance.
# Obviuosly a battery, etc...