Questions regarding self-constructed calorimeter

I'm trying to use an self-constructed calorimeter (as shown in picture), which is constructed by two different size test tube and a stopper on the top.


I'm using this calorimeter to measure reaction heat as shown below


Mg(s)+2H+ (qq)→Mg2+ (aq)+H2( g) ΔH1


MgO(s)+2H+ (aq)→Mg2+ (aq)+H2O(l) ΔH2


[My question is : ]


1. Considering the first reaction would produce hydrogen gas, how do I prevent explosion and heat loss at the same time.
(I could make the hole bigger on the stopper. But this would create a greater heat loss while doing so.)
2. If I want to speed up the process, should I use concentrated hydrochloric acid without dilution.
Is there any safety issue I should think of?


[![enter image description here](https://i.stack.imgur.com/qNlXl.jpg)](https://i.stack.imgur.com/qNlXl.jpg)
[![enter image description here](https://i.stack.imgur.com/lUbUd.jpg)](https://i.stack.imgur.com/lUbUd.jpg)
[![enter image description here](https://i.stack.imgur.com/fMhzO.jpg)](https://i.stack.imgur.com/fMhzO.jpg)
[![enter image description here](https://i.stack.imgur.com/tdYfe.jpg)](https://i.stack.imgur.com/tdYfe.jpg)


