Per Section 5.4.c of the Official Rules, gusset plates shall be in contact with and glued to at least two frame members in contact.  Since the base plate is not considered a frame member, gusset plates would technically not be allowed.  However, you may be able to make a wall member work as long as the triangle’s vertical and horizontal dimensions are at least 1 in. per Section 5.3.a of the Official Rules.  To attach the model to the base plate, we’d recommend drilling holes or notching the base plate so that frame and wall members can be slotted into the holes and glued for a secure attachment (Section 5.7.c of the Official Rules).