In the past when acidifying my container blueberries, I would often make a "peat tea". Soak some peat in water and use that (or better, dilute the concentrate up to make batches of less acidic water to irrigate the container). In the case of citrus, you would only need to bring it into the neutral/slightly acid range - 5.5 - 6.5 (with my blueberries, they needed to be down in the more acidic 4.5 - 5.5 range).

I found a more complicated recipe (involving making a concentrate) here - http://www.pitcherplant.com/more_info/peat_tea.html

Alternately, I usually use a soil acidifier (Espoma makes one) that you can sprinkle on top. This is what I use on a bunch of acid lovers - https://www.espoma.com/product/soil-acidifier/

It is a non-aluminum-based soluble acidifier. Not sure what it is exactly based on their ingredients list but it is probably ammonium sulfate with other things. There are rate directions on the label for use on potted plants.

You could also consider garden sulfur, but that is more long term/slower-acting and usually used for plants planted in the ground.

As a note, I have seen GW posts about Miracid (which Scotts/Miracle Gro changed the name of to "Miracle Gro Acid-Loving Plant Food" (or similar name)) and years ago, it used to contain an acidifier but no more. It is basically just a fertilizer targeted to acid-loving plants.

What is the pH of your water? I have been adding 0.75 to 1 Tbsp. Vinegar per gallon to my liquid feed solutuon and it has made a huge difference. One reason is that my tap water measures 9.2 to 9.4

My tap water measures 8.7 to 8.9. Do you add a white vinegar or apple cider vinegar? 1 Tbsp per gallon?

Any table vinegar will work similarly - you just want the acidity level to be somewhere around 5-7%. 1-2 tablespoons per gallon of water.

Just yesterday, I was reading that Cottonseed Meal is a good way to acidify container citrus. Any body know if this is true?

It is true that cottonseed meal has an acidification factor. But it is pretty slight - IOW, you would have to add a significant quantity to realize any sort of measurable lowering of the pH. Then there is the issue of dry organic fertilizers - like cottonseed meal - not being processed properly in a soil-less potting mix. If not sufficient populations of soil microorganisms present in the potting mix - and there very rarely are - the organics cannot be converted into plant usable nutes and the stuff is just wasted.

Cottonseed meal is helpful in maintaining the proper soil acidity for inground plantings but is not particularly efficient in actually lowering soil pH and doubly less efficient when attempting to use it in any sort of container gardening situation. There are several better and easier ways of accomplishing the same thing.

Vinegar is probably OK for hobby container growing, but because it is an organic acid its life span is pretty short. Another pH help would be to irrigate with rain water.

Many of the hydroponics companies use phosphoric acid in their 'PH Down' liquids. I use vinegar because my system is DTW (drain to waste aka: a container) and as long as I figure the input water going in is around PH 6 it's probably good enough.

I also found this link https://www.hydroponics.com.au/organic-acid-for-ph-adjustment/ (near as I can tell an Australian trade publication), which recommends measuring PH from the run-off, not the input water.

I have also read some posts by home hydroponics growers making their own PH-Down formulas with sulphuric acid (pre-diluted battery acid from the auto shop not from old car batteries). As the website will probably set off most corporate browser filters, I won't link to it but the search terms to get that page are "hydroponics ph down battery acid forum".

If regular white vinegar is too expensive you can buy 10% acetic acid "cleaning vinegar", use 1/2 as much (as there's 2x the amount of acid). My grocery store sells it for effectively the same price /bottle as table vinegar.