Going to do 20 days of kasina practice in September.  It is my first time. Is there anything I can do to make it most productive?

I have about 3-6 hours a day to practice this summer. About 10-20 days out, I am going to crank it up to about 6-8 hours a day.

I was planning on just doing kasina about 5-7 hours a week to get some muscle memory around the mantra and the rest of the logistics worked out but spending the rest of time doing insight practice. I wonder if more kasina or other concentration practices would help?

I do not have a concentration background. I am not sure if I have ever hit any jhanna's. I have worked hard at annapanna as a base for insight on retreats. It did have psychedelic effects.

My long term goal is to progress on the path of insight. My short term goal is to master this technique as much as I can by the end of the retreat.

Where are you in the insight path, if you don’t mind sharing? Or do you know? 

I think at least stream entry but I try to keep a skeptical scenario as well.

It happened 4 years ago after my third geonka retreat. I recall review cessations, immeadiate access to a&p at all times and feeling like a regression occurred when the next path started. It was a pivotal point in life that changed everything. It has held up. The problem was that I had no idea what happened until I read Daniel's book 1.5 years later. Memory is not all that reliable and was not having any more frutitions so I acknowledged that I could be deluding myself. I figured 2nd path would confirm.

In January/February, I had clearly struggled with 3c's, had an a&p event followed by 5 days of poor concentration that broke with equanimity. I had a fruition about 3 times in similar circumstances: Meditating to the edge of sleep with very subtle vibrations/low energy and then coming back with lots of energy and a&p vibrations. I dont observe the doors clearly but there was something of a "taste of death" that was familiar to the first experiences. However, it didnt feel like a review period followed or much has changed. It is possible that I just fell asleep and jerked awake. Haha. Maybe a near miss... Around this time due to life issues, meditation went from 3-4 hours a day to a very distracted 1 hour a day so that makes for more uncertainty.

In the past few weeks, I got serious again and i am floating between 3c's and dark night with no ultra clear a&p event. It felt like 3c's broke to mild a&p raptures and then mild loss of concentration.

TBD basically. Haha

Okay, then, you should have a good basis for a concentration practice, however the particulars of the diagnosis ultimately work out. Read through Shannon Stein’s Fire Kasina book and the other materials at https://firekasina.org/. Work on jhana as well. In fact, it is probably best to access jhana as a foundation for fire kasina. I recommend Leigh Brasington RightConcentration. https://www.amazon.com/Right-Concentration-Practical-Guide-Jhanas-ebook/dp/B017JJOXQ6/ref=sr_1_1?s=books&ie=UTF8&qid=1530933629&sr=1-1&keywords=right+concentration