1. Dilution of 10% SG of 1.73 as solid not powder. So for 5 litres use 500g borax by weight or if similar in volume 500ml. Maybe decant into smaller lots to get a sense of weight/volume : - may have scales :)
Since arrival the volume/mass is roughly comparable to water, so shall consider mixing by volume 1:10 with water. See also comment 2, 1:16 or 1:20 may be more effective.

Solubility

Decahydrate is soluble in water (1 g/16 ml at room temperature; 1 g/0.6 ml boiling water) or glycerol (1 g/ml at room temperature). Insoluble in ethanol.1
https://www4.mpbio.com/ecom/docs/proddata.nsf/(webtds2)/190308

2. Thicker would splash less but would it absorb as well.

3. Use brush only on top, and vertical surfaces where there is wood to be treated below to catch run off and drips.

4. lay on tarpaulin and could be useful on clean cotton duck

It should be applied at a dilution of around 1 part borax to 10 parts water. It is best applied to a moist surface and will travel through the grain of the wood – several coats are recommended. If possible timber should be dipped in a weaker solution (around 1:20) and allowed to soak. Upon drying crystals of borax may form on the surface of the wood and can simply be wiped away.
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