Goop is really unnecessary with those brackets. The hole spacing on the bottom of 5 1/4" drives was pretty much standardized just like it is for 3.5"'s, so you should be able to place those brackets right over the two holes on each long side of the horizontal surface of the tray, bolt it on, and bolt your 3.5" drive in between them. It's not rocket science.

Not *that* long ago it wasn't at all usual to get a free set of those brackets in the package whenever you bought a new (boxed) hard drive. If you know anyone with a "PC parts junk box" they probably have a set or two in it. Heck, I hate to think how many of those I've thrown away.