Plural modifiers are entirely acceptable.
Your doubt about the validity of the plural form comes from a rule in the past that noun modifiers had to be singular (apple tree, vegetable soup, rubber chicken) but today this is not an absolute and there are many examples of plural noun modifiers in everyday use, for example, parts departments, schools superintendents and options markets.
In Quirk et al., A Comprehensive Grammar of the English Language, the following situations are listed where a plural modifier may be used:
the singular form might lead to ambiguity
an Arts degree (a degree in the
humanities) as opposed to an art
degree (a degree in fine art)
there is no singular form of a noun (in pluralia tantum)
a customs officer
there is a need to denote variety
a soft drinks manufacturer [but] a
a topical issue comes forth, often in newspaper stories. Quirk cites
examples of Watergate reporting from
the tapes issue
the tapes compromise
the Watergate tapes affair
the White House tapes mystery and other examples, including jobs cut.
In your case, chemicals shelf might be used because of the variety of chemicals. However, I disagree about the potential for confusion. Taking the example of the electronic engineer, when could that ever be interpreted as some sort of robotic device? No such thing even exists. The same applies to mechanical engineer but in that case there's no alternative.