Dianes suggestion works very well against the beetles. We've used the product Tree Saver and managed to save all of our trees but one. Merp Inc. dba Tree Saver 17894 73 CT. N Loxahatchee Fl. 33470 (561) 655-6940
In order to get around the shell issue we drilled holes as specified in the directions that came with the syringe and solution, inserted short pieces of bamboo into the holes, sealed the outer end with silicone and injected through the dried silicone.
The reason for the sealant is to prevent the product from leaking back out of the tree. The solution is oxytetracycline Hydrochloride. I would imagine if you preferred you could also use brass or stainless tubing from one of the hardware stores.
I would also contact the company to get accurate dimensions for the shell material.
This worked reasonably well, maybe not as well as the shells, but it worked. Very Very important not to use the coconuts after treatment or allow anyone else to.