I admit that for a finished product 304 might save

Although 304 is not as corrosion resistant to salt water as 316.

 It was making a significant difference  
  but as i did get a quote about 18mths ago where 50off 304 3mm
plates where £13 each its still a large percentage of the
manufacturing cost the differnece does diminish as a percentage when
the sale proice is Jacked up to £100 - £200

But  on my latest quotes its only £4 for 3mm  and £7 on a 5mm plate
(difference between 304 & 316) But manufacturing costs are 3-4 times
on the 10 to 15 off batches.
 Like you  Nigel  im doing this using Works Contacts  but i would
love to be able to  order a  large batch of  say  100 off and get a
good price  as well as  try a few other  Stainless ideas

 I agree 304 is less resistant than 316    BUT i do think 304 is fit
for purpose and a backplate  its a very large lump that is rinced ( or
should be ) at least in a semi regular  way  and not a small intricate
thing thats going to Jam if a spot of rust does appear  nor is it
strapped to a boat and open to wind and salt spray 365days a year

atb

Duncan