Kofod Larsen Spear Chair straps
The dowels are long enough to fit loosely in the 4 slots under the seat frame. It is probably a 3/8" diameter dowel. And one side of the dowel is flattened a lot. This flat part goes up against the seat frame. Use a roll of Pirelli webbing. It goes around the dowel. And you cinch it tight. The flattened part of the dowel keeps the dowel from rolling in the slot and loosening the strap.
Can you send a photo? Or draw something? Also, is Pirelli webbing what's at the link below?
Also, what about Fagas Straps?
Fagas straps are pre-cut in the factory with the sheet metal embedded into the ends. They are designed for the later seat frame support system with continuous 45 degree groove cuts. Your chair has the earlier frame support design of three holes on each side, for which Fagas straps do not work.
Sorry forgot the link
The 2" Pirelli webbing looks right.
I can't seem to find a photo of the bottom of a Selig chair with the cinch tight straps right now, but this detail might help. You can see the end of the strap going down through the hole and coming back up. What you can't see from the top is the flat sided dowel underneath that the strap loop around.
It looks like you are thinking the dowels are about 2" long and that there would be one dowel per strap end. This is incorrect.
There are 4 dowels total. They fit into the 4 shallow grooves on the underside of the seat frame. The flat part sits up against the seat frame. Thus the dowel can not roll in the groove.
Does this make sense?
Look at the first image you posted on this thread. Shows the underside of the seat frame.
Observe that there is a groove along each bottom side of the frame; so there are 4 grooves in total. The through holes for the straps are inside these grooves.
The 4 dowels fit inside these 4 grooves. The flat of the dowel rests in the groove; thus the dowel can not spin.