Paul,
If you modify
skimmer cups, you may have issues with the volume of air, running through the
skimmer, lifting the cap off. They are kind of engineered with all of this in mind.
With that said, you could modify by cutting some height off of it, but you MAY have to adjust the hole or holes on the cover to allow for more air to escape, therefore reducing the liklihood of such an event.
The OTHER thing that I'd do, is drill the collection cup and modify it to have a hose, for overflow that drains into an empty soda bottle or something. The reason I say "I" would do that, is that once again, you've modified the
skimmer (straying from the engineering design) to a point where it doesn't have the same holding capacity. I'd rather have it fill a bottle, than to overflow back into my tank or on my carpet!
Dave
You are getting smarter. 
at my basement tonight. Give me a call.
V-series Productions 
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