You may well find that a series 2 spinnaker “bag” will fit, or alternatively you could have a bag or net made to measure. There are plenty of firms who will make canvaswork to order, and the attached photo shows a pair of spinnaker nets on a series 1 that I owned a dozen years ago – albeit that I used them for stowing a limited amount of cruising kit rather than the spinnaker.
Alternatively you could stow the spinnaker in a bucket. That works well, and the bucket will double as your emergency large capacity bailer, which (in a series 1) you are very likely to need in the event of capsize.
Oliver