In Trove there was a Harry Slack of Northgate, Brisbane who married (to Ivy Chalk) at the City Tabrnacle in Oct 1932 but this appears to be too late for the Harry about whom you speak if he was 20 in c.1905. I guess this would be the Baptist Tabernacle in the city area of Brisbane. He may well have lived later at Eagle Farm in Brisbane as in 1948 there is an advert for a 'dairy clearing sale' in ;account Mr Harry Slack, Reservoir Road, Eagle Farm'. The wedding notice just refers to his parents as 'Mr & Mrs Slack' so maybe his father's name was Harry. Did your father-in-law ever live around Northgate, on the north side of Brisbane? There was also another Harry Slack on Trove but he was a much older gentleman who lived in the Isisford district and died in 1924. He would appear to have much older than 20 around 1905. Gloria