This is a remix from laird's watch strap and Lindsay Fowler's buckle. I added a latch and I used a piece of 1.75mm filament for the pin on the buckle.

I have made the strap 20mm wide and only 1mm thin since I use Fillamentum flexfill 98A TPU filament, which is rather rigid for this purpose at 2mm thickness.

Printing these (I have used slic3r):

Strap: I suggest splitting the file so you print one part of the strap at a time. This reduces stringing.
Orient the strap sideways on the print bed. I printed at 020mm layer height with a 2mm brim. Make sure the loops for the pins are printed in a continuous way (see pic of screencap from slic3r). Some settings might make the loop part print in little bits.
I have modified the buckle side of the strap so that it incorporates a support over the hole where the latch would go. You need to cut this out after printing.

Buckle and latch: There are two files for the buckle: one complete and one split. I had some issues when printing the buckle whole so I ended up splitting it in two. printed in 0.05mm thickness. The latch is also printed in 0.05mm with a 2mm brim.
a short length of 1.75mm filament is used to hold the buckle to the strap and the latch to the buckle. The ends have been heated and deformed so the pin stays put.