A possible one-word synonym for "make (sth) constant" is "stabilize".
However, "stabilize" suggests that the thing being stabilized (in this case, the pressure) was unstable, which might not always be the case.
Your sentence could be improved if you extraposed what does not need to remain in mid position:
This method makes/keeps the pressure in a container constant when its temperature / when the temperature inside is less/lower than 10ºC.
I think "keep sth constant" makes more sense that "make sth constant".
Otherwise, if the condition was not constant and becomes constant by applying a method, "stabilize" could be a more appropriate choice.
ell.stackexchange.com/users/49058/Gustavson
It is grammatical to bring the word constant to the front of the sentence in that situation so that it comes after makes : This method makes constant the pressure in a container when its internal temperature is less than 10℃. or to place it after container : This method makes the pressure in a container constant when its internal temperature is less than 10℃.
Compare:
He wished to make known a feature of so-called "small clauses".
He wished to make a feature of so-called "small clauses" known.
P.S.
You can also substitute the verb keep for the verb make as user Gustavson says in his answer.
I think keep is better here than make .
ell.stackexchange.com/users/10264/Tᴚoɯɐuo
What about fix ?
This method fixes the pressure in a container when the temperature in the container is less than 10℃.
ell.stackexchange.com/users/40945/Diamond
What about " stabilizes "? to cause something to become fixed and stop changing, esp.
in order to keep it from becoming worse, or to become fixed and stop changing: These medicines stabilize your heart rate and lower blood pressure.
So, in your sentence, This method stabilizes the pressure in a container when the internal temperature is less than 10℃.
If the implication is that the pressure is otherwise yoyoing up and down, this should work quite well for you.
Alternately, you could rearrange your sentence slightly to avoid repeating words and bring "make" and "constant" closer together:
This method makes the pressure in a container constant when the internal temperature is less than 10℃.
This doesn't require a new term, instead cleaning up the text.
ell.stackexchange.com/users/16318/Catija
Opposing constant forces meet at a point called an equilibrium.
The single-word verb form of this is equilibrate .
This method equilibrates the pressure in a container, when the temperature in the container is less than 10℃.
ell.stackexchange.com/users/5144/LawrenceC
This method makes the pressure in a container when the temperature in  the container is less than 10℃ constant.   This is just an example I created.  In this example, the words "make" and "constant" are away from each other. Is it possible to rewrite this in such a manner that "This method makes constant the pressure..."? or is there a single word  as a substitute for "make .. constant"?
Is there a single word as a substitute for "make .. constant"?
ell.stackexchange.com
<phrases>
ell.stackexchange.com/questions/132097
ell.stackexchange.com/users/46924/rama9
"Stabilizes" could be used as a one-word term. However, this can be rewritten to clean up the text by bringing "make" and "constant" closer together, as in "This method makes the pressure in a container constant when the internal temperature is less than 10 degrees" When internal temperature of a container is less than 10°C, this method stabilizes the pressure. 
When internal temperature of a container is less than 10°C, this method stabilizes the pressure. Alternatively, to avoid repeating words and bring "make" and "constant" closer together, you could rearrange your sentence slightly. When internal temperature is below 10 °C, the method keeps the pressure constant.