I don't know if it will work on Windows but on mac os9.1 i found a way to get rectangulir terrain.
AND it isnt very tricky to do...
The way I found out how to do it was an accident but it happend.
To do it you MUST have a mouse roller.
1.Select terrain type (eg. tar water etc etc...)
2.Hold down left click and roll roller up and down while holding left click.
2+.Then let go of left click but DON'T click again and move the curser around the screen.
3.If terrain comes out behind curser without clicking so far so good.
4.The choose another terrain type and repeat process.
5.Do it with about five different terrain types.
6.Then press undo lots of times until chunks off terrain are there.
7.Choose a terrain type hold on left click and go over it if that all works...IT WORKS!!!
PS.Do process in a corner because if you put terrain on top of it it will be queer.
PPS.It is not perfect and it is hard to use but if you don't have a map hack that does square terrain use this.I c some perple are eager to try it-------->

post me if it dosent work.
Guess what, my friend found this long ago. Its what he uses. So ha. ur not only one who found it