Tweaks to laser-cut design for more robust assembly

[amcewen]

Paul Bullock, who takes PPE through certification as part of his normal work, took a look over our design (alongside the Prusa 3D print and the Kitronik laser-cut version).

He was generally happy with it, but had a couple of suggestions for minor tweaks to improve it - generally from the perspective of making the assembled visor more secure/robust.

The three things he suggested improving were:

• widen the spacing between the centre two tabs that hold the clear plastic part onto the headband, so it sits flush without smoothing out
• add a "mushroom" bulge to the tabs holding the clear plastic part on the headband, with a diameter of 6.7mm. That'll mean there's an interference fit with a standard hole-punch hole, to stop the clear plastic from coming loose
• alter the vertical slits where the adjustment strap feeds through the headband to provide an opposing tab so that the tabs on the strap better lock against the headband, to make the connection more secure

[amcewen]

Making the vertical slits into an opposing tab could also make the design better suited to die-cutting too (see #95)

[huffeec]

I've made these changes. There is a visor cut from this design in the events space.
If someone can show this to anyone taking these visors it'd be good to know if we can switch. The current design generates lots of tiny offcuts that can get stuck and need scrapping off the headbands, so it takes a step out of the cleaning process.

https://github.com/DoESLiverpool/covid19/blob/master/visor-designs/laser-cut/hybrid_4holePunchSpacingV4.dxf