I also found my original Strida seats to be a fiendish type of arse torture
I once did a 22 mile round trip and walked around the next day with people asking me quetions normally levelled at babies and their nappies (diapers)!!!!!
I bought a custom sprung flame saddle for my 1st Strida 3, & it transformed the bike I started to use it as more than a quick, clean city runaround, it got stolen in Belfast, I was almost as p***ed off at the loss of the seat as I was at losing the bike!
Got a replacement Strida and was so glad to get a trip to Christiania, Copenhagen so that I could buy a replacement seat.
When I got my Strida 5 the first real ride was around London and 2 minutes after returning sore-arsed, I was swapping saddles!
See it on my Strida page
Basically any seat which has parrallel seat rails will fit but the wider sprung type saddles such as the custom saddles available in many patterns and colours are amazingly comfortable, or even Brookes twin railed saddles will fit, (Wet the leather and then ride until dry to start the break in process). The wider one's will add to the weight (mine's about 5 lb! but worth every ounce!!!!)
To fit simply undo the pinch bolt, then lever off with a large screwdiver or tyre iron, ease the replacement over one side then lever into second side, Slide back or forward to suit (another advantage is that you can gain around 4" without moving the seat up) Replace the pinch bolt.
One slight niggle is that the Strida tool is a bit difficult to get in & out of it's hiding place under the seat (well with the custom saddles anyway)
Good luck, happy arsed riding! Let us know how you get on.