What is the significance of not making a garment from the spider webs?
Radak: Spider webs are not proper to make a garment from them for covering. So Their deeds will not cover (protect) them when the evil comes, for their deeds are evil deeds.
Malbim: Do not say that even though the outcomes that the philosophers derived are false, their axioms and thought processes are true, and they can be used to make a good garment, and bring from them proofs of the true Emunah. Many philosophers drew false conclusions for they erred about how to use their axioms, but their axioms helped for other investigations. These [deniers], their webs will not be a garment. Their proofs are from imagination, and not from intellect.
Why does it add "v'Lo Yiskasu b'Ma'aseihem"?
Malbim: Do not say that at least they themselves have a covering (excuse), that they erred in their intellect and investigations. All can see that there was no room to err. It is all deceit and forgery.
Why does it add "Ma'aseihem Ma'aser Aven"?
Malbim: Do not say that in any case, they follow the philosophers' creed and good Midos Bein Adam l'Chavero; they established wise etiquette. Really, their deeds are sin; man swallows his colleague alive, and their dealings are theft.