I usually have $1.5mn "idle" and have been most getting $70ish daily IB cash interest. Sold some boxes but found the ex-commission interest rate not much better (i roll every month) so stopped bothering. Might try to buy t-bills as vosotros proponeis aqui.
To park idle cash for 1-3 months without margin haircut, IB cash rate/SPX boxes/US-T ..IB cash rate is rather competitive
You have $1.5 for sure everyday at a minimum - does this amount swing or its fairly stable. is this for yourself or Entropy as a group (as a group maybe more predictable cash balance or less predictable not sure, just wondering how many variables there are that might affect this 1.5 mil from not being close to 1.5 mil. What kind of return on the $1.5 would satisfy you on a short term basis?