A foot is a (variable) measure of length. A board foot is a measure of volume - with an implicit third dimension. So you could just use the SI unit for volume

A foot is a (variable) measure of length. A board foot is a measure of volume - with an implicit third dimension. So you could just use the SI unit for volume. A board foot ~= 2.4 litres. This seems a bit silly though, as timber is not fungible in the way you'd expect a volume of something to be. As noted by sibling comments, you very much can buy timber in cubic metres - a convenience for costing / transport - but you're not going to buy timber without knowing dimensions of the individual lengths.