the big problem is that writing even a simple WM is
not
trivial. still, i would guess that taking twm and hacking it to
enforce window sizes and position can't be that hard.
twm would be an awful choice -- the code's completely unreadable
and surprisingly huge. There are plenty of ultra-minimal window
managers around these days that have far better code. Practically
anything written since 9wm (a pivotal app in the window manager
world). Hacking on one of those need not be hard at all.
http://www.plig.org/xwinman/others.html
The newer ones are generally nearer the top.
I wrote wm2 and wmx which have far too particular a look for the
sort of thing you want, but the code's not _too_ bad and I'd be
happy to answer questions (though I haven't the time to work on
any actual coding of this sort). They're in C++ with BSD-style
licensing. They're getting on a bit now though and there are
doubtless easier choices even than wm2 these days.
http://www.all-day-breakfast.com/wm2/
Chris