6x 8" Skyway non-pneumatic wheels (KOP)
ground flat with SBR Roughtop inclined conveyor belting from McMaster-Carr on them, attached with small wood screws. We ended up using
ginormous 54-tooth AndyMark aluminum sprockets on them. Next year, I'd really like to downsize to 6", (Somewhere I have math that backs up what Jeff said about those being a sweet spot, at least in terms of gearing) although one of the mentors wants bigger wheels. (I really have no clue why)