Does the outfit have to be a shroud, or can it just be jeans and a t-shirt and such? I don't have access to a shroud.

You don't have to use a shroud, but you should try for the closest option. The point is not dogma, it's to be in comfortable clothes that are loose and neutral. Jeans are restrictive (you wouldn't work out in that), time-dependent (it is relevant to trends and to a period in fashion), and mundane (so mundane that some workplaces forbid them, although they are considered "normal" clothes). None of this helps you focus on the gods instead of modern daily, petty life. You can look up online how to make a gown (the easiest sewing exercise out there) so that you have perfect and dedicated ritual clothes, or at least use a cheap cotton nightgown of the right color and without patterns.