A form suitable for use in an underground storage tank installation includes an upper portion sized and shaped to receive a street box and a lower portion that fits over and surrounds an upper portion of a riser on the tank. In preferred embodiments, the riser acts as a containment sump, a portion of the riser sidewall is double walled, and the bottom of the lower portion of the form extends to a depth below the top of the double walled portion of the riser. Preferably, the cross sectional shape of the form is circular and the bottom of the riser's lower portion is outwardly flanged, which serves to support the form during installation operations. The form has a stepped structure, with the upper portion of the form having a smaller circumference than the lower portion and a transition between the upper and lower portions forming the step.