Scotty to the rescue!!
The original canon base I had was foam...when I went with the resin base, I used Dap weldwood glue to adhere the arm to the base. Now that i see what you are working with I understand. I have to look at my Carlart P1 canon...but I think It is light enough to just glue that arm to your base. Definitly go the suspender route...just clip them to the bottoms like a pair of fishing waders..works great and totally adjustable. I went with figure #2 on MI's diagram of arm attachment. I liked the elastic ( yes, the kind sold at craft store) not the biggest velcro fan...always think that it will come apart at the wrong time. Purchasing a black bodysuit ( the kind worn under stormtroopers) is an easy and confortable thing to wear under the suit. It stays in place under the latex and breaths...also it looks fine at the armpits when you raise your arms ( allows for ventilation...it will get hot..oh, yes it will)
Now suit up ...let's see this bad boy in action