I have used Biller slip tips on fish to around 4 feet long, specifically the PK-312 mounted on a regular spear rather than on a pole spear. The solid slip tip slips off after penetrating and is held by the stainless cable, not a flimsy flopper pin. The other real advantage is that the fish is held by the cable with the tip at 90 degrees. Thus the spear is no longer in the fish and won't get bent.
Is in this image:
http://www.abbiller.com/wp-content/t...ccessories.png
Available here:
http://www.abbiller.com/products/pole-spears/
Depending on your spear shaft thread, you may have to use the regular speargun tip, the ST-312:
http://www.abbiller.com/wp-content/t..._shaft-tip.png