I can say from experience that they are pretty easy to make with simple tools. Steel packing bands are a practical and effective source of raw material, easily cut and formed to fit. Since the clips themselves are completely hidden when assembled, appearance is immaterial. Convincing replica rivets are readily made from common nails.

(You need to Login to view media files and links)
Replica handguard with packing-band clips, common nail rivets. Finished to match original stock.