I think the gel idea is an excellent solution. There are huge choices in that area and they could be cut down to fit. Best case would be a filter ring with a slot. This would be glassless so weight would be easier to manage. May I add Infra-red filter and polarizer to the mix?Hi Jake the strength is use is .9ND , but here i need a strong ND like that ,i combined it with the original ND .
check the filters here : http://www.tiffen.com/results.html?search_type_no=134&tablename=kodakaccessories
its a thin piece of plastic , 10 cm x 10 cm and just cut it out .