Went to Germany last week, first time I used the silly Strom panniers. I used two Gelert 30l stuff sacs in one pannier - £4 each from Amazon if I remember correctly. One stuffed fairly full, one around 1/2 full fitted a treat.
The other side got the good old bin bag (white thick plastic ones from Aldi/Lidl) & it too worked well, I got more in that side by packing a shirt rolled around pants and socks and putting each "day" roll into its own tesco bag & it was good for all the dirty stuff as the holiday wore on. I just used two Penny Market bags for the return trip - similar to Lidl bags, stuffed full of dirty washing.
Big rucsack in the top box - almost filled it, just enough space let for a 500ml bottle of water, my disc lock, a spare pair of gloves and a bit of old yoga mat to use as a seat protector for the tie-down straps. Used the rucksack to carry electronics/chargers, kindle, paperwork & a full change of clothes for the overnight ferry from Newcastle.