Being a former vertical caver, I use a system of prusik knots to ascend
my mast. The advantages of prusiks is that they are inexpensive and
nearly foolproof. You must have a seat harness which you can tie from
a piece of 1" webbing as a so-called "diaper sling". You must also
have a chest harness attached to the seat harness. You have a prusik
on each foot and one on your chest. You sit relaxed hanging from the
chest loop, raise each foot prusik in sequence then stand up while
sliding the chest prusik. Sounds complicated but I have climbed out of
a 1000' pit this way.
I personally do not like mechanical ascendars because they allow
foolish people to get in trouble. They seem too easy to use and I have
seen people make serious mistakes with them.
Remember, you MUST have both a chest and seat harness and they MUST be
attached to each other at the front, otherwise you can end up hanging
by your feet.