the stainless steel plates (called “coupons”) should mimic as exactly as possible the actual surfaces encountered in your equipment, regarding stainless steel quality and polishing. Regarding the dimensions, they depend on the surface you are going to swab; usual are 100 cm² (10x10 cm) or 25 cm² (5x5 cm), so the coupons should be slighly bigger than that (12x12 cm or 6x6 cm). This is necessary because if you swab the coupon from end to end without lifting the swab, you will get a better recovery than the actual sampling in the equipment, representing a “better case”. There are comercially available coupons, but they are quite expensive and, on top, are not the same surfaces you find in your equipment. In my opinion the best (but most tedious) way is to contact the manufacturer of your equipment for samples of the equipment surfaces, which will have the same material and polishing as the equipment, can be requested with a materials certificate, and are often supplied for free from sound equipment manufacturers, but it can be cumbersome to get in contact with the right person to get them.