Today I tried to lock the laser to the folded cavity with SR560 and succeeded to do that though it lasted about 10 sec.

In addition, I estimated the finesse from the linewidth of transmitted beam.
First, I measured the cavity length with a ruler and it was about 82 mm.
Then I scanned the laser frequency and measured the linewidth of the transmitted beam.

The free spectral range can be estimated as
f_FSR = 3e8/(2*83e-3)=1.8*10⁹ Hz.
And the linewidth was about 1.4*10⁵ Hz.
Therefore, the finesse can be calculated as F = 1.3*10⁴.