~ 1/4". I used a nominal sized plexiglass, but its listed as that.
Another seller on eBay is offering MDF motherboards cut for Spirit packs. Sorry if the seller is also here on the forum and I didn’t notice (i’m still figuring out who goes by what screennames). I bought one and it fits great but I will need to buy longer screws as the original motherboard screws for the cardboard/foam are just a little short for my liking.
*Update, I was able to use #4 x 3/4" wood screws to secure the MDF board to the Spirit pack