Questions about DIY acrylic tank

Well I'm considering building a tank out of acrylic rather than glass, but I will know for sure whether it's financially possible when I go get prices from my local acrylic supplier.

My plan is to go 8x4x2 (lxwxh) outside dimensions, but keep the water level at about 18-20" so my aro has a bit of leeway when he gets spooked. I am thinking of building it all out of 1/2" acrylic and using weld on 4.

My questions would be:

1. I watched the diy joey build videos on youtube and he added one piece at a time, welding each individual piece and making sure that everything was square every time he added another piece. Can you assemble the whole thing, making sure everything is perfect, and then do one round of weldon? Or is it more advisable to do it the way he did?

2. I plan on using one sheet as the top brace. Would this, combined with the dimensions, water height and acrylic thickness, be strong enough?

Another question I have is whether or not it matters if the top bracing sheet is on top or inside the aquarium. I've read that 1/2" Should be enough. Do you have experience building tanks KAWAMIKIE?

I've read weld on 4 works good for 1/2" thickness.

There's about 10 different companies in Edmonton that I have got quotes from. I've narrowed it down to sabic and EM plastics. The only problem with EM is that I want to do a full lid and they don't cut out holes, so I would have to do it myself and I cant say I trust myself not to crack a $400 piece of acrylic. lol