Ascham tells us that a bracer serves to save the archer's arm from the "stripe of the string" and his doublet from wear. It also keeps the string "gliding sharply and quickly" across the shooter's arm, thereby making a "sharper shot." Even so, Ascham believes it best "to give the bow so much bent, that the string need never touch a man's arm, and so a man need no bracer."
Where they are shown wearing bracers, the archers of our period wear what many modern archers consider a very small one. One can imagine that professional archers would have a very good idea of the exact point where the string might contact their arm. One can also imagine that soldiers might find it a matter of macho ego to "need" the least possible amount of this sort of protection, with the result that the smallest practical bracer was worn.
Ascham argues against having any sort of decoration on the bracer, such as "nails" (presumably metal mounts), buckles, or even aiglettes on the laces. Any sharp or raised surface is a problem. The nails can cut the string and buckles and aiglettes can "raze his bow, a thing both evil for the sight, and perilous for the fretting." Yet the lead archer in the Luttrell Psalter picture of an archery practice wears a studded bracer.
An example surviving in a private collection and illustrated in Dress Accessories provides a suitable design for a bracer. This example is made of one piece of leather and fitted with copper alloy mounts: a buckle, a strapkeeper, eyelet reinforcements, and a strap end. A cheaper version might have lead alloy fittings and/or assembled leather (i.e. the straps sewn or riveted to the main body).
A second surviving example excavated in York consists of a stout piece of leather pierced to take separate straps that are prevented from coming through the holes by the simple expedient of cutting the ends of the straps into wider "shoulders" that butt up against the edges of the slit. This example had an iron buckle (probably once tinned) that, due to stitching holes on the leather strap to which it is attached, have no correspondence with any other part of the piece, appears to have been re-used from another item, possibly a shoe.
A speculative, extremely cheap bracer might be a leather piece with a thong passing through holes at the edges of the guard and simply tied on.