Spiling isn't rocket science: any sailor with a steady hand can use a pencil or marker taped to a stick to approximate the curve, which will eventually be sanded smooth & padded anyway if the job's done right. Line each transverse cradle member up directly below the hull (or above it if the boat is lying topside-down), make sure it's level, and carefully trace the hull curve onto the wooden beam. The curves are specific, since they vary along the length of the hull, so be sure you have the transverse cradle members in the right place... sometimes it's easier to build the entire cradle, gently trace the curves where ya need 'em, then make your cuts. Double up beams if you want the extra support, you can cut each beam separately, then nail 'em together and sand 'em smooth till they fit the hull exactly. Padding can be strips of closed cell foam (sleeping pads for camping), thick neoprene, vinyl flooring material, etc., etc., glued on or secured in such a way that no nails or staples can scratch the hull. Paint all exposed wood for long-lasting protection, and use large stainless steel eye screws for tiedown points. Nothing wrong with well-designed & constructed wooden cradles, they are functional and offer a cheap alternative to welding, plus you can usually find all the wood you need in a jobsite dumpster, clean lumber scraps just begging to be used for the purpose.
Worried about a flat surface?
As I see it, concerns about trailers revolve around fiberglass damage by rollers and "oversprung" trailers knocking the Styrofoam blocks loose inside.
I'm lucky to have a bunk-bed, from which I can rest the Sunfish on a comfy mattress "nest". Alternatively, roll-up mattresses are available at Amazon. They compress while rolling them up, later expanding for use.
'Course, swim-noodles come in five-inch diameters.
What did we do before swim-noodles?
I appreciate you bringing the method to light.