While there are more elegant and comfortable solutions out there, we send rubberbands with every ProShield and TXRX Shield shipment because they are easy to find, cheap, and can be used to create straps of various tensions. What follows are instructions to create the combination that seems to work best for the ProShield.
Start with four #33 rubberbands and arrange them like so.
Lay a single rubberband over the end of the double bands.
Feed the two loops through each other, creating a square (aka reef) knot.
Pull until you have created a Square (aka Reef) knot.
Move the unused fourth rubberband so it lays over the double rubberband loop as in step 2.
This time, the end of the single loop goes under the double rubberband loop then through the other end of the single loop.
Pull to tighten the knot. You may need to unroll the double rubberbands afterward.
You can repeat this procedure with other combinations of single, double, and triple loops to get a fit that is best for you.