when i built my Katzuma i looked for a 750/1100 seat for ages but couldn't find one !!! so in the end i got one off a 400, they are basically the same but not as deep as the bigger bikes, when i fitted it the 1100 side panels where too long so had to find a pair of 400 side panels which are shorter. worked out ok in the end, so maybe a 400 seat can be extended at the base to give it the depth of an 1100 one ? by the way when i looked their where 3 400 seats to choose from at the time.
what it looked like on the bike.