The issue of outsized feet and the resultant odyssey in search of fins to fit them has proved to be a perennial problem in all forms of swimming and diving. Although manufacturers sometimes argue that their customers can squeeze their feet into the largest-sized fins they make, they don't or won't publish the internal lengths and widths of their foot pockets in centimetres and millimetres so purchasers can really be sure they match the dimensions of their feet.
The lacing solution was certainly practised by Russian spearfishermen, who often fashioned this workaround in their workshops at home because they had little or no access to the diving equipment market outside the country. Russian and Ukrainian fin manufacturers even supplied a couple of models with factory-installed laces and eyeholes:
The Australian Eyeline range of full-foot swim fins claims to offer fittings up to UK 17-19, which the company equates to US size 19-20; see
https://www.eyeline.com.au/Shop/Product.aspx?pid=EF4K&cid=351. However, the key measurement isn't the shoe size. It's the foot pocket's internal length and width in centimetres, which are 31.5 × 11.7. Would they match your foot dimensions, without your toes spilling out of the toe opening too much and causing an unwanted fulcrum possibly leading to a tear in the side walls of the toe opening? So determine the dimensions of your size 15/16 feet in terms of centimetres of length and width. If those Eyeline fins offering 31.5 cm internal length work, there's an option. And if so, don't reject them too quickly because of their association with swim training; in an online article entitled "My favourite kit", Australian professional underwater photographer Pete Atkinson was very satisfied with that style of fin.
Read more at
http://www.divernet.com/gear-features/p300505-my-favourite-kit-pete-atkinson.html. There are plenty of online videos showing how Eyeline-style foot pockets can be bonded to a blade to create a home-made monofin. Maybe something similar can be done to create freediving bifins?