In Boston, we play a more physical game of hot box.  Some people play
by the same contact rules as Ultimate, but for us, anything that is
permitted in basketball is certainly allowed for us, and then some.
Choose the permitted level of roughness to suit your tastes.  For us,
anything short of outright tackling is permitted.  We try to watch the
fouls on the thrower, though.

The basic rules are:

1.  The goal is a box anywhere from 4' by 4' to 10' by 10'.  To score
a goal, you must have two feet in the box.  We usually use shoes to
mark the corners.  Spray paint works well, but you have to reapply it
fairly often.  Very small cones would work, too.  If you play with a
small box, there will be a lot of trying to jump into the box to
score. 
2.  Stall count is 5.  Anybody can stall from anywhere.
3.  After a goal or turnover, you must clear the disc.  We have two
lines on opposite sides of the box 10-20 yards away.  We play shorter
to minimize running.  Also, the defense usually doesn't contest the
clear.  For more running, you would try to shut down the clear and
play 20 yards.  Clearing is determined by first point of contact.
4.  Arguments are permitted and even encouraged.  For a goal to count,
one of the scored upon players must agree it's a goal.
5.  Just because something is illegal in ultimate doesn't mean it's
illegal in box.  Setting up a post, body contact, some travelling,
deliberately throwing a pass off a defender, stalling from 10 yards
away, all these things are legal in box (although you of course are
permitted to outlaw anything in your own game--there's no universal
set of rules).
6.  Some people play you can't guard the box unless someone is in it.
Others play that even the offense can't go into the box except to
catch the pass (sort of like the paint in basketball).  These people
are sissies.

Since scoring percentage in box is so much lower than in ultimate,
it's more acceptable to try for spectacular plays.  In ultimate, in
general, it's poor strategy to try for plays that look great.  Our
team's offensive strategy is to score without having anyone say,
"Great catch" or "Great throw", since every great throw is a hair away
from being a turnover.  Of course, try those great throws in practice
and in less meaningful games, so your throws improve, but in any
situation where your first three goals are to win, win, and win,
exercise some caution.

Box is good practice for a few reasons.  One, it develops quicker
throws, even if you don't use the same actual throws in ultimate.
Stall is 5, people are open for split seconds only, fewer cutters.
Two, it helps develop throws against the mark and against the zone.
Three, you develop better body control, especially if you play with a
small box.  Many of the plays require you to make a huge effort to
land your feet in the box.  It teaches you to develop awareness of the
line and how to land in-bounds.  Four, you learn how to throw dump
passes better.

I'd like to recommend another game as well.  Play 4 on 4, field about
40 yd by 20, 10 yard end zone.  Play normally, except it's make it,
take it, and no check after the goal.  Play games to 3 or 4.
Turnovers or pulls or goals are put into play immediately where the
disc is.  So you can score in one end zone, have a cherry picker
sprint to the other endzone for a huck and a goal, and return the huck
for a third goal and the game.  If you have exactly eight people,
maybe you could play games to 4, best of 7.  If you have ten or
eleven, play games to 3, loser sits, take one or two of the players
from the losing team.  It's a good workout, the games  seem more
meaningful than 5 on 5 game to 15, and it's fun.