Reveni is brilliant for the size and sits on the hot shoe. For slide film though I'd want a proper Sekonic with spot metering — the latitude is too tight to guess.
Late to this but see my blind-test thread in Equipment — short version: the phone app hangs with the Sekonic except in backlight. If budget is tight, spend the money on film and use Lightme.
If you do want a real one, the Reveni Labs spot meter is tiny, made in small batches, and reads like a meter three times the price. Mine lives on the strap lug now.