I never did like plunger molds period. To me you really cannot get them tight enough to prevent hot plastic from getting on you. Having said that try putting the mold in a table top vise, wear goggles, gloves and a heavy apron for your protection. Wear shoes instead of flip flops also.

Then, as soon as you are financially able invest in a good aluminum mold. You will never regret it. Look at the safety tips at the top of this forum. Read it often. It will save your body a lot of pain.