Your best bet is plexiglass. As it is VERY hard to get glass to silicone together and hold water indefinately. The tanks we buy at the store are machine built, and it is next to impossible for a human to replicate that. (you gotta factor in exactly smooth/level glass edges and a perfectly level working surface when working with glass and silicone)
Neither is cheap..but plexi is going to be in the end....after you think about how much $$ you will spend on silicone after the seal doesn't hold on the glass over and over lol.
But TBH, building a tank is going to cost you just as much as buying one..unless you plan on making more then 1-2. Because of all the supplies you have to buy up front. If you plan to make more then 2-3 tanks...then yes it would definately be worth it.
You will also have to make a lid...that's the hardest part. As if the tank is not perfectly square and/or you don't cut the lid to exactly match the tank's dimensions, once it's complete...you can have gaps.
This is the site I used to build my 22g divided tank...
http
/www.garf.org/tank/buildtank.asp
I thought it was great, very detailed and informative.
The only thing I didn't agree with...I don't think you should use anything less then 1/4" for any home built tank...the sides tend to bow out if you do.
Those you buy at the store that are 1/8" are made of 1 piece molded..not 5 seperate pieces. Doesn't seem like it would, but it really makes a difference in stability and strength.
I've sortof not got a lot of betta space. Well, I potentially do, but its finding a tank to suit. I was planning on a 2g betta cube type thingumybob. I worked out each edge should be 20cm, but thats about it. Well, whats best to make it out of (on a low budget), where would I buy the stuff (in the UK), any other dohickey I should know.
. I'll have to try and get hold of some nice big tupperwarey things 
