Here are a few boxes that traded today in the SPX, fyi. Dec 22 4000/5000 box 992.10 So you are loaning 992.10 and getting 7.90 in interest over about 13.5 months here if you buy this box. Dec 23 4500/5000 box 980.05
The boxes themselves will trade on a fairly tight market, even though the individual legs are quite wide. I would start with a 1 lot and change the price until filled. You can also watch time and sales and if you see a box trading at what would be the bid, you can try and enter an order at that price.
why would you ever buy the box? do you finance at better than 70bps? And if so, isn't there better use for your capital?
If truly risk-free, a profit can be leveraged to any arbitrarily high level without the risk of bankruptcy, so the return appears to be a matter of choice. In reality however, the rate of return is limited by the gross margin set by the brokerage, which effectively caps this type arbitrages to annualized returns below 10%. There are still rare and low volume mispricing that lead to significantly higher returns but they are expensive to capture.
As long as the return is greater than your cost of funding. typically it’s not as the market charges libor and retail trades fund at libor+x