Preparing a concentrated metaborate stock solution from borax and sodium hydroxide works and is convenient. I'm not sure if the alkalinity isn't high enough (pH >11 @ 10% w/v) that storage in PET plastic bottles might be a problem. Polyethylene should be OK, but is gas permeable, so carbon dioxide could diffuse into the solution and lower the pH. I store stock solutions in glass bottles and haven't noticed them attacking the glass.
The 7g Na2B407.10H20 (borax) +1.5g NaOH (sodium hydroxide, lye) is approximately correct and will give the equivalent of 10g NaBO2.4H2O (or Na2B2O4.8H2O), i.e. sodium metaborate octahydrate, or sodium metaborate 8-mol, or Kodalk. If a formula calls for the tetrahydrate (also called 4-mol or dihydrate when represented as NaBO2.2H2O), the conversion factor is 1.00g tetrahydrate = 1.35g octahydrate.
Alkali borates have many hydration states and crystalline structures, though in the end we're interested how much boron in the correct ionic form ends up in our recipes. The concentrated stock solution gets around that problem by avoiding hydration water altogether (and the variations in hydroxide strength can be dealt with qualitatively by "titrating" with borax to neutralization.
I personally like having and using solid reagents wherever possible, so I prepare kodalk as a solid. The preparation comes from US2886425, which has a nice summary of the stoichiometry of the reaction between borax and sodium hydroxide, and US1976299, which incidentally is an Eastman Kodak developer patent, but outlines the preparation of metaborate (although that form would have a degree of hydration between the 4-mol and the 8-mol species):
69.15g of borax (decahydrate) and 15.20g sodium hydroxide (purity 99%, 5% excess) are mixed in a plastic beaker as dry powders and 17.0ml of ice-cold DI water (5% excess) are added to the powder with strong manual stirring (a sturdy plastic spatula or spoon work well). The water is quickly absorbed and within a minute, the whole mass goes through a thick (molasses), thin (syrup), and crystalline (wet sugar) phase, releasing substantial heat. Stirring is maintained as the mixture thickens and almost solidifies, so that the mass is broken up into granules in size between rough sand and rice. If the mixture isn't stirred with sufficient enthusiasm, it will solidify into a rock (which i fact would happily destroy coffee grinders and blenders..). Once the mass cools to about 30C (80-ish F), it can be spread out to fully cool and equilibriate, and is then transferred for storage. This yields approximately 100g of sodium metaborate octahydrate. The excess of hydroxide accounts for carbonate and water content (addendum: the amount can vary from the stoichiometric 14.50g to +1-5% without reflecting on the buffering capacity of the final product, but some unreacted borax will remain dispersed in the solid), and the additional water makes up any amount that evaporates during the reaction. In any event, the final product might vary within a few percent, but is photographically viable and the preparation can be scaled up so that a single run will yield years' supply of Kodalk. Plastic utensils are used throughout to prevent metal contamination.
Please bear in mind this is dangerous, lye is corrosive and the reaction produces substantial heat. Wear appropriate PPE and follow safety practices. That said, the preparation is convenient and affords a stable product.
I use AR purity hydroxide, because I can source it cheaply. I believe food grade hydroxide is good enough, because it is specified for low heavy metals content. Iron contamination doesn't exceed 0.005%, which contributes less than 8ppm iron in the final metaborate. addendum: dissolving the hydroxide in the water and adding the solution to borax also works, but results in more mechanical losses and is overall more diffcult)