it might help if you posted year and model of Roo/Shamrock, since you don't have a sig line listing it.
because older models may have different canvas attachments, as the newer ones do.
i have a 2007 23SS and i don't know what white plastic pieces you're talking about.
pics also help alot.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC