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Calculating slippage for SPY

I am looking for a reasonable slippage model for SPY.

SPY of course is unlike any stock in terms of its massive volume. In addition, I would think the slippage would be somewhat constrained by the fact that the price of SPY reflects the prices of the underlying 500 stocks.

The average daily volume for SPY is 90M shares, or about 230K shares/minute. There is much greater volume BOD and EOD, so let's assume an average of 200K shares/minute.

Assume I want to buy 5,000 shares, or about 2% of the average SPY volume/minute.

Which of the following slippage models would be most realistic?

1) Take the average of the high/low price (ie, price obtained = (high+low) / 2

2) Spread the 5,000 share purchase over 5 minutes, using the same formula as above (ie, (high+low)/2 ), but trading just 1,000 shares each minute

3) Use a volume-weighted average price (VWAP). But is there a way to get the VWAP for an individual bar?

And would I need to use a different slippage model for shorting SPY?