A large Carradice bag would fit all of that stuff; something like the Nelson longflap, so you could strap the towel on the outside, underneath the longflap cover with raingear if necessary.
Or you could get a seatpost mounted rack and a trunk with drop-side panniers (Topeak makes one.)
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"I feel like my world was classier before I found cyclocross."
- Mandi M.