Heer46 is offering conversion kits for 15mm halftracks:
From their website:
Its time for the Allied boys to pimp up there M3 Halftrack with “Planen” The set comes with four planen and some stuff for the M3 Halftrack. You see on the picture a Battlefront M3 Halftrack and the four planen.
Edit: Scale corrected. Thanks DamianR.