to my knowledge the plastic on the bottom is the transfer medium of pressure and weight of the tank onto a stand, it also adds some protection to the glass and gives strength to the corners. the plastic on the top of the tank, if a center brace is present stops the glass from being pushed outward (bowed) by high water pressure, usually only needed on tanks 40+ gallons large.
if you have the means you can always substutate the plastic framing with wood (both top and bottom) and/or at the top of the tank, you just use a peice of glass siliconed to the sides to act as a center brace.
or if you can build the tank to manufactured aquarium dimensions, then you can special order the plastic frames from a manufacture or LFS
. i had to replace the top brace on a 55gallon tank a long time back, i went to my local LFS
special ordered it for 16 bucks.