Adhesive For Clickbond Nutplates
I am using various Clickbond products in my RV-7 build, enough to keep a supply of the recommended Clickbond CB200-40 adhesive on hand. Depending on how many nutplates you plan to install, it is understandable that you may not want to purchase a 40ml cartridge ($43 approx including UPS shipping from ACS.)
But as suggested above, the small packets of CB200-40 adhesive are available if you only plan on a couple of nutplates. The cost of them adds up fast if more than a few nutplates are involved, justifying the 40ml cartridge price. By looking at the usual online sources including the auction sites, you can obtain the cartridge applicator gun for $20 approx. A mixing nozzle is not required and for small jobs wastes too much mixed adhesive inside the nozzle itself. Just squeeze out what you need from the cartridge directly onto a piece of foil or paper, and mix the two components by hand like you would with the small packets.
There are undoubtedly other consumer grade adhesives that might work, if economy is the only consideration.
I elected to not use anything other than the recommended adhesive, especially for Clickbond nutplates. My reason is that the nutplates are normally put in a place where dealing with them later would be difficult if the adhesive failed, at least they were in my RV-7 project.
That said, Clickbond CB200-40 adhesive is a methyl methacrylate based adhesive. Methyl methacrylate adhesives are used in critical applications, like human joint replacements and various industrial structural assembly processes where high strength is required but riveting or welding is not possible.
If I were looking for a substitute, I would look for something methyl methacryate based (it is different from crazy glue, and definitely is not epoxy/flox/cabosil although I have no opinion on whether those would work).
An example is the 3M Scotch Weld DP8410NS series. But there is no significant economy with using that instead of the Clickbond brand.