Mounting screws for horseshoe backplate

The Cox .049 product-engine (horseshoe) backplate can be mounted with small wood screws obviously.. but what's worth doing.. is worth doing right... especially when it comes to securely mounting an engine in an airframe. That said.. the horseshoe backplate has X4 holes for mounting radially. The holes all have a .125" (1/8") inside diameter.. so your screw "body" (threaded portion and shank) can be that diameter or less. The screw "head" obviously needs to be a larger diameter.. but ideally between .180" and .200" diameter. Any "washers" used (flat, lock or otherwise) also cannot exceed that diameter.

I'm being a tad conservative with these measurements.. but take a look at the photo below; where I'm using 5-40 threaded hex-head "cap-screws" for mounting my engine to a bulkhead/plate.. and you'll see that there's no room for a washer that's any larger of a diameter.. than that of the screw-head.