The cheapest way to mount a gopro to your wooden speargun. Make a trip to your nearest Ace Hardware / Best Buy store and pick up the following items:
From Best Buy:
GoPro Tripod Mount = $6.99: www.bestbuy.com/site/insignia-...
From Ace Hardware:
2"-3-1/2" Corner Brace ( Item # 5290473)= $1.79
Rigid Tie RTU2 (Item # 5607742) =$.99
2 Screws for rig =$0.08 each
2 Fasteners bolts for rig = $0.14 each
1 Stainless Steel Screw for GoPro mount = $0.28
1 Stainless Steel Fastener for GoPro mount = $0.14
Total Cost: $10.63
Let me know if this worked for you guys :)