Actually, the surface skimmer came with a couple of adaptors which I
am sure will fit the intake tube you have. On mine, it actually
replaced the whole tube and the plastic 'grill" at the end. Not sure
if this makes any sense, but the skimmer attaches right to the ribbed
tubing for my filter.

You might want to check out your local pet store to see if they have
one and then try to see if the dimensions will work for you before you
drill any holes. Seems like a whole lot of work and retrofitting for
under $20.