Hi Temp Silicone gasket material (available at most auto parts stores) works very well and is easily removed. http://www.delcity.net/store/Red-Sil...FacDOgodVT8AOQ

Do Not use Hi temperature epoxey unless you want the modification to be permanent. JB Weld Hi temp is great and almost impossible to remove, It can be removed with considerable effort and painstaking detail work with a dremel.

Step 1. Clean mold I prefer Comet cleanser and plenty of hot water and a toothbrush.
Step 2. Once dry, if water was hot should be almost instantly. Carefully and neatly fill the desired area with silicone - do this on both halves of the mold. A single edged razor blade really helps if you have excess on mold faces.
Step 3. Dust surface of mold and siliconed area with talcum powder
Step 4. Install hook(s) close mold I like to clamp with a quick grip clamp and let cure per package directions
Step 5. open mold, remove hook(s) trim if necessary with a razor blade or exacto type knife.

I have molds modified following this method that last over 1000 pours. I've also made the mods and after a few casts decided I did not like what I had done and picked the mod off the mold. A little tube of the stuff will last a long time with proper storage and will do a lot of mold modification.

Removal is simple, use a dental pick, bodkin or something similar and peel the plug off the mold.

Alternately you can clip and remove the collar barb etc but I find the little bit of time this takes is well worth the effort.