Here are a couple bottles i happen to have ...
More comments in a minute.
Nice looking sauce and label! White would look better, but if not available...
I do not recommend using boiling water. Water can get inside the cap threads, maybe compromise the HFH seal. Water will be under the shrink band and can further harbor Nasties. This topic has been discussed previously, make your own choice. If using a heat gun, using the fan attachment helps.
The capsules look like pre-formed, like what is used on wine bottles. Shrink bands are usually flat cut.
Check out bottle selling sites, they often have accessories like shrink bands and orofice reducers. There are shrink wrap companies that can do any color and any size....for an order of 20-50,000.
I checked out a couple bottle sites that used to have a wide variety of band sizes and colors. Unfortunately, their supply is severely reduced. Supply chain issues, i have been dealing with this with other packaging things. I could only get white tins instead of black, and white droppers instead of black. Oh well...if they want the product, and post-2020...color shouldnt matter.
Couple other things to keep in mind, get bands that come down far on the neck to cover the fill line. No matter how hard you try, every bottle has a different fill line. You want that covered so people dont bypass the lower fill line and only buy the fullest bottles.
There are professional heat tunnels available. Professional packers (co-packers) will have labeling and heat tunnels integrated into their processing lines.
I dont know if you are processing the sauce yourself, if this is a legal sauce or a hobby sauce...but as a small batch 'artisan' sauce, hand made, etc, people do not expect perfection in the packaging, only in the product.
I make my same exact hot sauce recipe and hand apply the labels and Bestbuy dates, and shrink bands. Some are a bit wonky, some shrink bands are a little more wrinkly. Think about the farmers market soap maker with twine wrapped around each bar. Every twine bow is different. Embrace it!
Good luck in your search!