One small surface in the house set aside for God.
A shelf or a table top is enough. A cloth in the color of the season, a crucifix, an icon or image, a candle.
Change the cloth with the liturgical season so the house keeps time with the Church.
Pray there at a fixed hour so the place and the habit hold each other up.
Attention follows space. A house that has nowhere to pray will find that no one prays, and a house that has one corner set aside will find that children use it without being told.
The Eastern Christian home has kept an icon corner for as long as there have been Christian homes. The Western equivalent went by many names and was normal in Catholic houses well into the twentieth century.
It faded with television and open plan rooms more than with any decision, which is why rebuilding it is usually a matter of furniture rather than theology.
Entirely free. Start with a candle and a crucifix.