You can get the threads you want from sailrite.com. If you are going to machine stitch I would suggest no heavier than V-69 and needles to match if you are going to use a domestic sewing machine.
Using a cover over the trampoline is something I believe strongly in, both to save the tramp from UV exposure and to keep the birds away (or small buggers who wants to use your boat as a trampoline at the club).