If you have a model of the system and solve it in Explicit Dynamics, you will be able to measure the reaction force at the fixed end of the hollow tube. Do not leave the hollow tube out and apply the fixed support on the back of the plate, that will not be the same reaction force at all.
For an elastic hollow tube structure behind the elastic rubber/metal plate, you can't remove the hollow tube from the system and just impact the rubber/plate and get the same reaction force. Below I explain why.
The way to think about this is springs in series. There is the spring rate of the rubber Kr, the spring rate of the plate Kp, and the spring rate of the hollow tube Kt.
The equivalent spring rate Keq is computed from the springs in series equation:
1/Keq = 1/Kr + 1/Kp + 1/Kt
The peak force on the hollow tube occurs when all the KE is converted to Potential Energy PE stored in those springs.
PE= 1/2*Keq*Xmax
where Xmax is the distance the moving mass travels after impact before its velocity becomes zero.
You can solve for Xmax by setting PE = KE.
But first you need to know Keq. Use a Static Structural analysis, apply a small displacement Xu to the face of the rubber and output the reaction force Fu. Now you can calculate Keq = Fu/Xu.
A good estimate of the maximum force Fmax = Keq*Xmax.
You can see that if you leave Kt out of the calculation of Keq, the estimated force will be much larger.