this idea is for a modular and compact kit to carry in the bag.
(Something looking a little less improvised than a piece of paper)

you can attach every accessory you may invent. for example coloured diffusers or reflectors.

this is created on Nikon D40, but I think this would be fine also for other cameras.

Step 1: Material

- plastic cards black(for reflectors) and semi-transparent(for diffuser)

- self adhesive silver/golden (or whatelse you can imagine)

- flat aluminium bar (section 2cm x 2mm)

- self adhesive velcro

you will also need scissors, pencil , a saw(for metal) and a file.

Step 2: The Universal Support

Cut a 11cm long piece of aluminium bar.

Measure and divide(with pencil) the bar into 4 parts each long 2-1-4-4 cm.

Bend the sections as shown in picture (angles +90/+90/-45 degrees)
[well as you can see I wasn't able to bend 90 and 90, but the important is the sum of 180 degrees]

The first section of the bar must enter the hot-shoe flash support of your camera, but 20mm is too much, so the first section is to be filed on the sides.

Then apply the self adesive velcro on the last section.

Step 3: Cut the Plasticard

Trace this shape on the plasticard.
the radius is 8cm for reflectors and 10-11cm for the diffuser.
the angle is about 78-80 degrees

then cut away the vertex (triangle of 2,5cm)

Step 4: Velcro

Attach the velcro on the shapes as shown in picture
(of course not the same kind you already put on the support arm)

Step 5: Reflective

Attach the reflective adhesive matherial on the other face of black cards.

Step 6: How Mount It and Final Result

1)open the pop-up flash
2)insert the support arm from behind under the arch of the pop-up flash
3)insert the support arm in the hot-shoe support
4)then attach the accessory with velcro.