Random parameter for lottery in public access

I do a lottery, identify the winner randomly, according to the formula

W = Np * E

where W is the ordinal number of the winning participant (each has his own)

Np - the number of participants

E is a random parameter known and open to participants at a particular moment! The lottery is held online using live video.

I don’t want to use a simple random code, because the participants will have any doubts about honesty.

For example, E may be a fractional part of the pound sterling rate to russian ruble from the CBRF website on the date the winner is determined. If there is a course of 82.5681, then I multiply the number of participants by 0.5681, round off and get the winning serial number.

The advantage of this option is that info about the sterling rate for everyone is in the public domain, in a reliable source and none of us can influence it.

The disadvantage that does not suit me is that the course is updated only once a day! Sunday is not updated at all, when I am going to announce the winner. Need more often, that is, something more volatile, galloping.

I do not want to take the rate of bitcoin or stocks on the Forex market, people do not have confidence in this.

In which Internet resource should I take a similar parameter? Random.org uses atmospheric pressure? I need not only me, but also the participant on his PC to see the numerical value that I use for calculation.