An ECDSA private key is simply a 256 bit integer between 0x1 and 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364140. So all you need to do is combine those random numbers so that the result is in that range. The result is your private keys. I'm pretty sure that however you combine those numbers will still be safe, so how you want to do it is up to you.