Here's some examples of the phrase "a/one hell of a" from a dictionary:
‘They have asked Santa for bikes because they have a hell of (a lot of) cycling to do.’
‘Assuming we get any takers at all in this mad scheme, it should be a hell of (a lot of) fun.’
‘With these aircraft coming to the end of their lives, the cost of replacing them is a hell of (a lot of) money.’
‘Even its biggest advocates would have to admit that it really is one hell of (a lot of) hot air blowing slowly round the internet.’
‘It would need one hell of (a lot of) earthworms to digest that sort of quantity, and the beds and borders aren't getting any fuller.’
‘It will take time and effort and money too, though a hell of (a lot) less than buying one legally.’
‘And I have to admit that there is one hell of (a lot of) good source material I could be using.’
‘The reason the girls are outperforming the lads is because they work a hell of (a lot) harder.’
As shown in the dictionary, the expression is "a/one hell of a...", and indeed "a/one hell of" is always followed by "a".
In all the examples cited here, "a/one hell of" is followed by "a lot of"/"a lot" which is then followed by an uncountable noun, a plural noun, or even an adjective.
I have excluded those examples where "a/one hell of" is followed by "a" and a countable noun. And I have bracketed "a lot of" or "a lot" to see if you can remove the phrase without making the sentence ungrammatical.
Does any one of these examples work without the bracketed "a lot of" or "a lot"?
If not, I wonder why is "a/one hell of" always followed by "a", be it part of "a lot (of)" or of a noun phrase containing a singular noun?