Here it is a quick change filter holder using a cheap lens hood a piece of 5mm thick black foamex and a little black electrical tape and a piece of welding glass for a big stopper (a real filter is optional)
It is very light proof thanks to using the bayonet mount fitting for the Hood

Well the lens hoods for the 70-200 and EOS-m have arrived today from Hong Kong and I must say the 70-200 one looks as good as the original and flock lined inside to boot and together cost about a fiver delivered.

The EOS-M hood was £1.89 delivered crazy cheap

I also have received my 2 pieces of welding glass for big stoppers £1.20 each delivered

Now all I need to do is a bit of hacking and glueing and I am set.

Here is how it was done and all for less than £4.00
Measure from the back of the hood mount location on the lens to the furthest thing you need to avoid in this case the screw in filter.

Using a bit of ingenuity scribe a line around the hood here the knife was held down with one hand and the hood rotated

hacksaw the end of the hood off and sand it down remembering to cut the correct side of the line.

mount and check you got it correct

use the hood as a template to scribe a circle around on a filter sized or bigger piece of plastic about 5 to 6mm thick

should look like this when cut out

glue in place after determining the rotation so it doesn’t look skewed when mounted I made small matching scratch to line the hood and plate orientation.

 

lightly press down so the front edge of the hood and plate are flush

put some tape on the ends of the filter

fit onto plate and then mount on camera

Neat easy and very quick to mount and remove just needs a twist on or off.

Thats it made now I just need to see how many stops that welding glass is and mark it on the glass for future use

EDIT

The particular glass I have is rated as #8 in welding terms but worked out nicely at 10 stops.
It is important that you work out by test exposure what the light transmission of the glass is as it can vary a lot as its designed to protect the eyes from an electric arc not for photographic work I have had glass with 5 stops difference with the same rated welding number.