there is custom sling plates but they are ali so may be a bit heavy if your only looking for extra length.
https://custom-sling-plates.myshopify.com/
he may be able to make them out of delrin or any other material . you could email him .
or buy any of these and modify them https://www.google.co.uk/search?q=ri...w=1366&bih=662
or
you can buy thicker recoil pads . http://www.jsramsbottom.com/category...il--butt-pads/
or a shotgun extension slip on . above link also.