"I wouldn't be without these inexpensive, durable and useful packs of 100 stainless steel washers.
For holding down tarpaulins and felt outside, I use stainless Turbo Ultra countersunk wood screws with washers under the head. This copes with even strong wind and prevents the covers tearing. It is much more convenient than nailing battens down because it's so easy to unscrew and reuse everything.
They may not look much - but certainly get the job done."