bonding plexiglas

There is a special chimical glue that we use at the shop called Weld-On #3. You'll find it at a plastics shop and POSSIBLY at your local mariner shop. I have been told that Home Depot carries it, but I haven't seen it in my local HDs.

It has about the consistancy of water and cures in about 30 minutes.  When you buy it, best to buy a syringe to apply it in a controlled manner so you don't get curtains and weakspots. It's a chemical weld for them so after polishing and a bit of extra elbow grease, you may not even see a seam anymore (depending on quality of the install)

However it sounds like you are building a serious SPL system. Such is the case, I would also consider reinforcing the seams with a sandwich of steel strips and bolts using silicone to fill in any gapping in the bolt holes, etc.

Ganbatte ne!

Being the "best" will depend on your skill and point of view. Personally, I would heat form a large sheet of plexi and reinforce that, but that isn't the cheapest option (rather one of the most expensive ways really). Another favorite of mine is plastic welding. Real welding, but on plastice. Then drill and reinforce on that again. (Needs the most skill)

As for using #3, use the syringe to apply a thin layer of the glue to the bonding surfaces. When bonding, the key here is to not let bubbles enter the project as it is curing and while being applied. Tiny bubbles will affect the strength. A good option after bonding, will be to drill holes about every 8-12" and use metal strips to reinforce the bonded areas. Use a good plastic filler to fill in around the bolts while you bolt the reinforcements together.

Ganbatte ne!