I just started practicing hand cut dovetails and have been using a
square to mark the baseline. I would like to make a simple marking
gauge, but I think I may be missing something. They seem very simple to
make, but when I search online I'm finding some fairly expensive prices
for such a simple tool. It seems to me that if the pin or blade is
secure and can keep a consistant distance from the base, it would work
fine. For example, a dowel with a blade attached to one end and some
sort of sliding block (something like the collar used to hold weights
on a weightlifter's bar) would work just as well as anything else.

Is there anything about marking gauges I am missing?

Thanks,
Richard