I would like to start out by saying that I love the glass top flower boxes in the 1 on 1 arena, so much so that I've tried to duplicate it on my own. I am a novice map designer however and can not seem to duplicate this effect, I've tried adding the box under the floor (which it cuts a hole in the floor so that the player drops into the box. Then I tried to put a sheet brush on top of the box which gives the effect, but the sheet is not solid, The player still drops through into the box. Can anyone give me some pointers on how to achieve this awesome effect?

Fister?

make a thin square that covers the hole. approx. 1 to 2 units in height. thats what i figure would do the trick and give it the transparent effect. Just thinking about it not actually trying it.

It's not a sheet..it's an added brush (non-solid if I remember well )
Select the up/down surfaces of the brush an set the properties (only) to:
2-sided / translucent and portal; that should work

I tried using a solid, semi-solid and non-solid added brush but it adds the "walls" of that brush back to the world so that the flower box isn't under the floor and you see "void" on the edges of the glass top.

Yeah don't use a sheet cause I can't find a way to make them solid reliably. For invisible floors make yer brush say 10 units high an 2 units smaller than yer hole in the ground. Then select the top of the brush... and select matching brush (shift-b) and then surface properties, then check transparent if ya want it opaque or invisible if ya want it glass. When you were seing those voids it's because the invis is on the outside of the brush not the inside, so poor runeed can't grasp it hehe. Long Live Rune! Respond with any needed clarifications 'Cause my descriptions are convaluted .

ps : at the top bar of the surface propeties box it should say "x sufaces selected" where x is the entire outside of yer brush... so if you have a square hole you should have "6" surfaces to make invisible, not just the top and bottom : sides too .