Unless you are at a bank, you can’t lever this position. So why are you bothering trying to earn 20bps and be locked in for several months? (You will pay your 50bps bid offer if you have to get out).
I'm not understanding anything, not any one half sentence, of what you are saying in this or your previous comment. If you are questioning the motive, please rest assured that I and many others desire to leverage their portfolio at low rates via box spreads. Likewise, many counterparties also desire to earn interest with spare cash. That's by definition what makes the money market, and by implication the market of box spreads. I asked some very precise questions regarding the mechanics of options box spreads. Please let me know if you have any advise regarding my questions. Thanks.