MathGolf, 32 bytes
{kî-⌠ *_£(∞k£+'_*û/ ï*\n}û\/k* k
Explanation:
{ # Loop the (implicit) input amount of times:
k # Push the input-integer
î- # Subtract the 1-based loop-index
⌠ # + 2
* # And repeat that many spaces
_ # Duplicate this string
£ # Pop and push its length
( # + 1
∞ # * 2
k # Push the input-integer again
£ # Pop and push its length
+ # Add those two integers together
'_* '# And repeat that many "_"
û/ # Push the 2-char string "/ "
ï* # And repeat it the 0-based loop-index amount of times
\ # Then swap the top two strings on the stack
n # And push a newline character
}û\/k* # After the loop: repeat 2-char string "\/" the input amount of times
# Push a space
k # And push the input-integer
# (after which the stack is joined together and output implicitly)
