Do not attempt an acrylic sump until you know more.
1/8' is not thick enough for anything deeper than 8'. Use 1/4' material for your project.
Silicone may hold. Weld-on will hold. A well prepped weld-on joint is as strong as the panels it was made from.
A hand-held circular saw is pretty much guaranteed to result in a weak edge with lots of micro-fractures. Use a tablesaw and a blade with a negative rake angle (the teeth scrape through the material, not gouge), at least 80 teeth, and a triple chip grind tooth pattern. The usual wisdom about keeping the blade barely through the material is also reversed (along with the rake angle). When cutting acrylic, you'll have less melting of the cut edge if the blade is as high as possible. You'll probably have to sand after cutting. Use a block and a shooting board to hold the block 90 degrees to the edge. Any rounding of the edge will cause bubbles to appear in the joint on drying.
Your best bet is to find a local plastic supplier who stocks 1/4' clear acrylic (just about all of them) and ask them for six panels precut to the dimensions you need and prepped (sanded or routed) for gluing. They will only guarantee the accuracy of their cuts to 1/4' accuracy, but if you ask nicely and explain what you're doing, you can expect to actually get what you ask for.
1/4' acrylic is actually 6mm, which is slightly thinner than a true 1/4', but if you calculate the thickness in your plan as being 3/16', you'll find it easier to use weld-on 4 (it's a very watery solvent and there will be small protruding edges that you can run the glue applicator along).
Any joint you're gluing up should be parallel to the ground. The glue will immediately run out of any joint that's vertical. I made a jig to hold two acrylic pieces together at 90 degrees. Works okay, but I'll design another to be more adjustable.
If you want to use a panel for the top, the plastic supplier will also be able to cut a smooth opening in the middle of the panel. 1' wide edges on all sides will be sufficient for tanks up to 48' long and add a 2' wide center brace for tanks longer than 30'.
You can also avoid using a panel for the top, just use 1' strips. If you use this approach, the corner joints are the most important part. Put the strips on the long sides first, then take four 3/4'x2' pieces, and glue one under each corner so that it will support the strip you're going to add on the short sides. Then attach the short sides. If the tank ends up being more than 30' in length, put a 2' brace across the middle of the tank (just glue it on top of the edge strips).
In either case, don't apply any weight to the center brace (make a sticker so everyone will know) as you'll pop the joints holding the center of your long sides to the top. This is the voice of experience speaking here.
Regards,
